Switch out the fluid to ATF+4 if you haven't already. ATF+4 was specifically introduced to reduce converter shudder/chatter when DexMerc didn't cut it and ATF+3 proved even worse.
Alternately, try adding a tube (or two) of Lubegard Instant Shudder Fix (part #19610). If one tube helps but doesn't quite do it, add a second. Just about every parts chain carries it, and one of the few "magic fixes" I'll actually recommend to customers. It's safe for use with all transmissions and fluids except CVT, dual-clutch, and Type F. If you're using Type F--DC's hi-perf recommendation for the A904 and A727--it may be part of the problem.