Could be a bad torque converter. Had a cavalier once that would stall out at red lights/stop signs.. turns out it the converter wouldn't unlock. Not sure if you can unplug the lockout on the 700r4 or not, should be able to just to see if it goes away. There is a plug on the side that I have the TCI lockup kit hooked to on my built 700r4 but it has been awhile since I messed with it, especially since it works.