Fixing 1994 Buick Lesabre Transmission Problems

When my 94 Buick is cold and I put in it in drive or reverse it bumps, but won't go forwards or reverse for about 10 to 15 minutes, till it warms up. Then once warmed up it goes into gear and works perfectly until it cools down, then same over again. I changed fluid and filter and no difference. What do I do?

By Richard from Huntsville, AL