Peavey 5150 combo NEED HELP!

[sdre]

Hey guys. I'm working on a friends peavey and there is an issue with the distortion channel. When played at loud volumes it breaks up and sounds like a motorcycle.
......I bypassed the pre-amp by hooking the guitar in the loop return and the ......problem is gone.

.....The bias is reading 47 Ma
.....Voltage is reading around 500
.....Just replaced tubes as well. power and pre.

Any suggestions would be greatly appreciated!

Thanks in advance!