1996 Jimmy no shift ??bad trans??

I just got a 96 jimmy that I am concerned about the transmission. When you start it, the transmission works, it will pick up all the gears and shift normally for about 2 minutes. Then all the gears go out. No first and no reverse at all. If you sit and wait about an hour it will work again but it slipps then does not work at all. It you let it sit overnight it will work again for the 2 minutes. The previous owner told me that the jimmy had randomly died a few times on them but always restarted. All the wires are connected. The Jimmy was well maintained and a 1owner unit. It has 140K miles and is an automatic with 4wd. Any help would be great.. Thanks.

Any trans codes, is the fluid burnt, if no reverse and slips this is an internal major issue that requires a probable overhaul.

There are no codes. Also the Service engine light is not on. I am going to flush the fluid and see if that helps. I found sevral used transmissions in my area for $250-350 for 110K mile and up units.