hello all, I have a 91 Lexus Ls 400 135000 miles, nice car and drives good in the winter time and at night time when the temperature is cool. But when it gets a hot like in the summer and i drive it for about an hour on the road or even when i run it for about an hour, it starts to loose power and jerk like its gasping for air.

When i put it in park to idle, it revvs itself between 0 and 2000rpm. If i turn off the engine and let the car sit until the weather cools off, it starts to drive well again. a few mechanics could not determine what is wrong. lexus dealers are having a problem determining what it is. i need a possible explanation so i can pass it on to the mechanics. thank you all.