rough patch between 2nd and 3rd gear
 

84 300cd rumbles at a very specific point between 1500 and 2000 rpm, before what i believe is the 3-4 shift, assuming i'm starting in 2nd. i don't think its the flex disc... it *feels* fuel related but I don't think that is the case. i can pretty much eliminate it depending on how i use the pedal. any ideas?