1996 Pontiac Bonneville

Im looking to buy a 96 bonnevile and the owner has a problem. The car runs perfect when cold but if its hot out u can go about 20 mins and the trans will drop to 2nd gear and stay there until cool again the car is in great shape and runs nice mostly hwy miles
i think this may be a speed sensor or trans sensor gone wacky or maybe the trans valve is stickin so I want to know if that sounds right or if there is somethin else. What would be a cost? I figure 2-300 bucks to fix if a sensor 600 if valve I scoop the car up for 600 bucks so its a steal to me but I want to make sure the tranny isnt completely failing