Engine Bucking And a Stalling During Outside Temperature a Greater Than 85 Degrees

This car has app 90.000 miles.She had a tune up and new catalytic converter at app 78000 miles. When I drive to work at 0530 in the morning in the dark car runs well with no bucking and a rare stall. When I drive home at 1730and it is 85 or better and sunny she will occasionaly buck after 30 or so minutes after starting the trip though the bucking is now becoming more regularly than randomly home. When I exit the interstate and come to the light and she decides to stall it shuts down smoothly as if I had turned the ignition to off. She will also stall randomly driving around town within minutes of use. She likes to stall in the middle of an intersection as you try to go through.The odd thing is if I go from the existing run position to start I can crank till the cows come home and she will not start. If I inmediatly select off then start she starts up as good as when she left the factory. I recently had the computer updated by the dealer for a hard downshift problem which was fixed for two weeks but has since returned. Thank You in advance for your time and expertise Doug Summers