What is the reading of the Watt. meter? The P500 will shut down when the cont. Watt. usage is above 320W. If the vantilation around the P500 is not adequate, it will shut down sooner.
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.....