I had two with same problems, 1 was actually completely dead. The other was like yours. Power on and LED on, ~30-60 minutes later booted fine for duration! Took them apart and nothing obvious. They sat for a few years and it made me sick since they were both in minty white shape. Finally contacted jean at Amiga center and he agreed to repair them both! A few short weeks I had the boards back! He said leaky caps were the blaim (near audio jacks) Seems to be quite common. Good luck.
