I've been repairing consumer electronics since I was in HS ('64-'66). I would unplug the AC cord to the amplifier and carefully, with the proper sized tool(s), remove the top of the cabinet and look for printed circuit board connections (plugs). I would then wiggle them slightly as I pressed them back in place. Then, re-assemble the case, connect it back up, and see if the fault returns. If the fault does return. get it back to Onkyo while it still is under warranty.
I have had an Onkyo TX SR505 AVR for years and a C-7030/TX-8020 (stereo) for over a year with zero problems.