I had that problem with my 7500 and it was caused by dirty AC power.
I thought it was a ground loop problem so I tried the cheater plug but no difference.
I bought a Channel Islands AC filter and all of the problems went away.
Not sure if they still make them but Emotiva makes the CMX-2 that should do the same thing.
Epson 3500 3D projector
93" Carl's Place Flexiwhite DIY screen
Outlaw 976 Pre/Pro
Outlaw 7500 for LCR & L/R surrounds
1- Emotiva Mini X-A100 for rear surrounds
2- Outlaw LFM-1 EX subs
Oppo BDP-105 Blu-ray/DVD/SACD player
Outlaw LCR (Snell Labs) for center channel duty
Tekton Enzo XL L/R
4- Emotiva ERD-1s surround speakers
Monster HTS3600MKII power center