I had the same problem in the past. The transformer in the unit has gone bad. Email service support at PS Audio. They are very good too! They'll send you another transformer which is plug and play. I was told that the earlier model P-500's had transformers that were defective. I hope this helps.....
ps500 shutting down, HOT
I own one of these and never had a problem with heat before, but I recently added a Musical Fidelity tri-vista 21 dac and after about 2 hours of listening, the unit (ps500) goes into it's protective heat shutdown. Also plugged into the unit is a Supratek Cabernet pre, Sony xa-777sa. I plug my amp directly into the wall. I will call ps audio, but I thought I'd check here as well to see if anyone has any thoughts