might just be the bands in the tranny. went threw 2 700r4's in me 88 before i put in a th400. the 700 and ur 4L60E are pretty much the same transmission, with similar parts being used. I doubt a computer will be able to read if the bands are slipping or not. by the sound of the power u r making, u r on the fine line of how much a 4L60E will handle. you might want to think about doing an 80E swap or a th400. correct me if im wrong but a converter wont cause ur tranny to slip, only the bands will cause this? idk if im right or not about that but just my $.02