Dear Tom and Ray:

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?

-- Jim

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

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, Jim?

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.