My 2001 C70 LPT 52,000 miles automatic transmission, seems to stumble or hesitate at constant speeds above 30 mph whether or not the cruise control is engaged. Unfortunately the only code that was set vapor recovery system code, possibly a loose gas cap??? My mechanic saw no other codes. He suggested I wait until the check engine light comes on again. This is really not fun to drive and a lot more than annoyance. Any thoughts? The tachometer does not seem to change when this happens. Misfire/No fire spark plug? Injector not pulsing? Transmission torque converter unlocking/locking? This one has got us all baffled. Not sure where to start? Any diagnostic tricks up any one's sleeve I could try?

Crusing Speeds not cusing speeds perhaps could be cursing speeds. Sorry for spelling error in thread title.