how can u tell??

how u can tell if a converter is bad? here is wat happen went to the track couldnt 60 ft for nothing mph was down from 92 to 85 this is a 4200 i got from my freind i traded him my 3500 i used to 60 ft in the 1.60s couldnt today then going down the track everything was going good intill i went to 3rd then it wanted to rev real high and didnt want to pull but driving it normal back thur the return roads everything is fine its does this at wide open this is the first time i ran it at the track with this 4200 any ideas