Engine/Transmission (1994 - 1998) Bucking

[Jfam]

I recently got my 94 automatic built to to ~500 hp (see signature). One of the unexpected outcomes of the build was that in the 700-950 rpm range it "bucks. " Though very little drive time is spent there (mostly parking lots), it is very annoying. The guy at the shop who did the build cannot find the problem. The shop who is upgrading the transmission to handle the extra power is going to look at the problem. We first thought it was the timing, which was set to 20. But when reduced to 16. 5 there was no change. Any suggestions would be appreciated...