Loosing power on hot days/Trips

Notice lately that everytime its 90 and I go 30 to 40 mins
down the road by car loose acceleration. Even at a stop
sign it will not take off like it is suppose to. I almost got hit
a few times. Check engine light is not on and by the time I
get to the dealership it's back to normal after sitting for
20mins waiting on them to inspect it. They told me that
they cant find anything wrong with it and that if it happens
again to come back. My car has been maintained at
200,000 miles and drives great during the fall and winter.
What should I do to stop this issue if no one can tell me
what's wrong with it