There are a few things you can try.
1. Set the Oppo to LPCM instead of bitstreaming.
2. Try turning your receiver off and back on while the problem is occurring.
3. Try a different HDMI cable.
I would suspect the the HDMI cable is the likely culprit but it may be some weird handshaking thing as well.
I did not look at any tube preamps for the same reason that Car and Driver is not reporting on buggy whips. - David A. Rich, PhD
The audiophile reviewer will pick the most warped sound as his reference for sonic purity. - GranteedEV
Receiver: Marantz SR8002 Speakers: Paradigm Studio 20, Studio CC-490, Studio 10 Subwoofer: HSU VTF-3 Mk3 Sources: Oppo BDP-93, Xbox360 TV: Panasonic TC-P50V10