1990 Buick lesabre when cold transmission slips 2nd going to drive when it warms up it becomes better

What color is the trans fluid? Have you replaced the fluid and filter lately? If not might be time to do that.

and after a day or two drain out some more through the filler tube. Do it once or twice until it's nice and red......don't let anybody tell you that a drain and filter won't help. It may not work but it's a cheap first step.

P.S. don't put any stop leak in but a pre-draining condition a few miles ahead of time is ok.

I have found that adding a can of CTC really helps older transmissions.