I have a mystery problem. My car has done this twice in 2 years now, always in 90 degree heat and after driving 100-150 miles on the interstate with the cruise control set. The temperature gauge indicates all is well there. Then the RPMs drop to 2000 or less and if I continue to try to give it gas to maintain my speed, it will die. I can let off the gas and most times it will drop to 500 RPM then jump back to 2000 where I then can give it gas and just keep it at 2000 RPM to try to limp home. Then while trying to make it home sometimes it will start driving normal again for a while. Then the problem started again with the 2000 RPM problem. No check engine light or trouble indicated anywhere on the dash screen or gauges. Mechanic is stumped since there isn't anything to read and he's unable to recreate. Only has happened on the interstate, never in the city driving. Help!