I really do not know how else to explain it. This is a 1992 "L" 3800 and 4T60E trans with 125k miles. Never any problem , fluid is full and looks/smells new. Fluid and filter changed about 4 years/40k miles ago. When I check the fluid there are usually 2-3 small bubbles in the fluid on the dipstick.

On rare occasions (usually about five miles from a cold start) I will stop at a light (usually for 30 second-1 minute) and when it turns green it feels like it is in neutral. Push on gas and engine revs but no motion. A few seconds later, low engages and everything is fine therafter. Once going it shifts normally through the gears both on slow acceleration and floored. Not sure if I would call it a slip since is no forward motion, more like does not engage at all but felt fine coming up to the light and while stopped.

Just took out and tried to make it happen but could not. Has happened maybe four times in a month.

Has never done more than once in a trip which may be 30 miles or more. 90% of time everything is fine but since this is our trip car I need to know that it is unlikely to fail completely.

Any ideas what is causing this ?

Possibly a sticking solenoid. My daughter'* 91 GP had a similar problem (a different transmission, but similar workings.) Talking it over with a mechanic, he suggested to give Seafoam Trans Tune a try and see if that freed it up. Put a can in back in December, and it'* been working fine since. Then, change fhe fluid and filter...it'* due.