engine "stumbles or bucks" under both light and heavy acceleration. dont know history of vehicle. has been doing this for over 1year however has gotten extremely worse in last two weeks. seems to idle and decelerate with out problem. usually only happens under any kind of load like merging into traffic, take off from stop or even just going up a grade. is almost undrivable at this point. need help quickly it is my wifes only transportation to and from both her jobs.