Problem started this week, was in ohio in cold weather, when car first was started it went to a very high rpm, had to shut the engine down and restart to get it to idle normal. later that day it stalled coasting to a red light, started right back up, drove it to NC with no problems, got here and it did the same thing when i started it the next morning, very high idle rpms, shut it down, restarted it and it was fine, drove it 20 miles and it stalled at a light, restarted , no problem..I also have noticed that on interstate hills, it either has a misfire problem or the trans is not shifting down from overdrive without several trys, mash the gas and it drops a gear and is fine, i drive mostly at high speeds and long trips, 70/90 and 500 mile trips, not much city driving. what is causing it to stall and after sitting all night it has this very high idle when first started, also the misfire or trans problem on hills at high speed when it seems like it is trying to shift down or the engine has a misfire, I cant tell which it is, thank you for any answer you may have, no check engine light comes on but have not scanned for any codes