I had similar problems with a refurbished HK AVR 125. It turned out to be an internal grounding problem. After a couple of trips to my local authorized HK repair shop. HK deemed it a lemon and sent me an AVR 130. Good luck-sorry to hear you have to go to the shop-I know the feeling and feel your pain.

M22's, VP150, QS4's, HK 630, HSU VTF3-MKII