i have a 1994 chevy 2500 diesel pickup. It has high mileage on it- 290k. when i first drive thetruck it drives great, but at times when it warms up, it slips out of od and will not get out of 2nd or 3rd gear. i had a transmission specialist look at it and corrected an error code 87, which meant that the brakes were being applied, but it is still happening. I love the truck, but hate the problem with no results. can you help me figure out what might be causing this?

Optional Information:
1994 Chevrolet 2500 6.5l diesel

Already Tried:
checked out by a transmission and diesel pro