Sounds heat related. If it "works fine for 2 - 3 mins" something is probably heating up and expanding, and breaking a connection. I would look for a mechanical failure by probing around with a chopstick to see if I could either make it fail when it is working, or make it work when it has died. It has been subjected to temperature (expand/contract), as well as vibration related stresses, so I'd look for a mechanical before a component failure.
Also, if it was lying on it's back, look at the bigger components like filter caps or even tubes that would have been vibrated on a different plane than they were intended if it was standing in the normal vertical position. If it has caps glued in place, the glue can get brittle in the cold and allow the caps to vibrate free and cause a fracture in the solder joint that could certainly be a problem when warming up,