I have 92000 miles on my car and have owned it since new. Tranny fluid changed 3 times. At 75000 miles it started to, on rare rare occassion, seem to "clunk" into the gears as I accelerated. The issue would sometimes disappear for months. Recently I have noticed it happening on a weekly basis. Also just this week I was approaching a turn at a slow slow coast and when I accelerated upon turning the tranny slammed into gear almost like a neutral drop feel. This only happened once. If I shut the car off for a while to run into the store etc the clunk will not be there when I drive again.
Fluid has always been nice and red, no burning and no excessive deposits. The GM dealer has found it showing no codes. Any thoughts from you folks?

Welcome to the Bonneville forum. Please add your cars year and trim level in your sig.

The could be a motor mount, inspect them all. Check for anything loose.

Typical mount that might mess up is the one on the front of the trans. It'* the equivelent of a marshmellow, not good for much but lighting on fire..

Your symptoms are familiar though. If you had the codes read w/o shutting it off when it was slamming, most likely you'll find the Max Shift Adapt code. It'* something like P1811 (not that I know it off the top of my head ). When the trans sees something it doesn't like or the common pressure control soleniod starts heading south, this can happen.

Not that long ago someone posted about installing a shift kit and a can of seafoam trans tune. A few others followed along seeing good results. Personally it'* a bandaid, but the pcs is deep enough inside that the side cover on the trans has to come off which means trans has to come out.