unsual transmisson problem
 

hello forum i have a 1992 buick park avenue ultra the tranny seems to shift smooth but after like 5 minuts or so it just goes into neutral and does not do anything just stays in neutral after a lil while i shut of the car and restart it the tranny works nomaly can this be due to some kind of sensor relay or sleniod or is the trany bad the fluid seems clean in it any help would be highly apreciated and thank you ahead of time