•Q. I live in the San Francisco Bay area and love my 2003 Nissan Maxima. It works fine, except for a problem we began to experience last summer. On a trip to Las Vegas in hot weather, the car began to shift into third gear very roughly when we started climbing into the mountains. The temperature was in the high 90s, and the engine temperature went up, though not to the danger level. When the car shifted back into fourth, again, the shifting was very rough. On returning home, I brought it to the dealer. They could not find any problem, but we changed the transmission fluid, which was needed. We had the same problem going to Arizona a month later. High temperature, high altitude and a steep grade brought on the problem. Everything has been fine since then, until we went to Los Angeles. The same problem occurred when crossing the Grapevine in 100-degree weather. We can’t duplicate the problem for the dealer, since the weather is always cool, and we live near sea level. No one seems to know what is causing the problem or even where to look. If I get on the freeway here, I can floor the car, and it shifts smooth as silk. We’re going to Arizona again soon. Can you help?•

TOM: Yes, we can help. We’re attaching the names of rental car agencies near you.

RAY: It sounds to me like a transmission problem. And while it’s happening only in hot weather on hills now, it’s likely to start happening at other times sooner or later.

TOM: Going up steep hills in hot weather is when you put the greatest amount of stress on your transmission. Well, steep hills, hot weather and two mothers-in-law in the back seat. But generally speaking, that’s when a transmission is under the greatest load and is working the hardest. So, if something is wrong, that’s when you’re first likely to see it.

RAY: It could be something relatively easy to fix, like a lazy solenoid, a sticky valve or a software issue that can be solved by re-flashing your transmission computer. Or it could be the proverbial beginning of the end for this transmission. We don’t know.

TOM: If you don’t want to rent a car for your hot-weather trips and you’re reluctant to spend a lot of money on exploratory transmission surgery at this point, I’d have a mechanic install an auxiliary transmission cooler for you. That’ll cost you a couple of hundred bucks.

RAY: An auxiliary transmission cooler essentially is a small radiator that lowers the temperature of the transmission fluid. It’s often used by vehicles that tow things, because towing is very similar to climbing steep hills in that it puts an extra load on the transmission and makes it run hot.

TOM: If you’re lucky, and you’ve led a good, clean life, that’ll keep the transmission temperature low enough to prevent the problem from occurring. At least for a while.

RAY: Like we said, though, at some point, this probably will start happening in lower temperatures and on gentler hills. Good thing you don’t have any hills in San Francisco, right?

TOM: So keep in mind that even if our suggestion works, in all likelihood, you’ll still need more serious transmission repair — or replacement — at some point down the line.

•Q. I have an issue. OK, not so much an issue, more of an argument between me and my fiance. I just bought a 2008 Dodge Caliber. My new toy has everything I love, including cruise control. I do a lot of highway traveling, and I love my cruise control. My fiance says it’s no good that I use cruise control all the time, and that it’s bad for the engine or transmission. Is he right? Is using my cruise control too often bad for my car — will I break my cruise control if I use it too much? If so, why, and when should one use cruise control? Love you guys!•

RAY: If you think of the engagement period as a sort of test drive, your fiance just backed into a tree.

TOM: Yeah. He doesn’t have half a leg to stand on here. Using the cruise control won’t wear anything out. In fact, because it helps you maintain a constant speed, it actually prevents a certain amount of wear and tear that comes from accelerating and decelerating more frequently.

RAY: The engine and transmission couldn’t care less whether the electronic inputs are coming from your right foot or the cruise control system. And neither should your fiance. So, tell him if he doesn’t offer more fact-based advice in the future, he’s going to lose his male automotive-pontificating privileges for the duration of the marriage.

TOM: Plus, highway driving is exactly the time you DO want to use cruise control — when you’re maintaining a constant speed for a long period of time, when traffic is thin and moving predictably, and when there are few unexpected obstacles that get in your way (like pedestrians, bicyclists, crossing traffic or aggressive squeegee guys).

RAY: The time you don’t want to use cruise control is in dense traffic, or in stop-and-go traffic, where you could be expected to have to slow down or stop frequently or unexpectedly. But even that’s a safety issue, not a wear-and-tear issue.

TOM: Right. In the old days, the cruise control used a separate cable that physically moved the throttle. So there were a few small parts that could wear out over time back then. But cars don’t have throttle cables anymore. Everything’s done electronically, through the computer. So there’s nothing to wear out.

RAY: So, suggest that the future hubby concede defeat on this one. And if he tries to argue that he’s still right because you’re going to use up your lifetime allotment of electrons, run.

Got a question about cars? Write to Click and Clack in care of this newspaper, or e-mail them by visiting the Car Talk Web site at www.cartalk.com.