Everything Steve said. My G61RSL did something similar, bad cap on the Backplane, G91 similar symptoms with a bad Power supply. Some of the caps may look good, but actually be bad; very slight slightly bulging can be present or none at all. just the start of your board level troubleshooting, good luck!

808v6, 818.3, G65, G61RSL, G91DH, G08, G98DH, 500v3, 504, 218-LPS, 218, 210, MS200
8000uSE, 5200SE, 5200HSE, DSP5200 (nonSL~fully rebuilt w/ Excel tweeters), DSP3300x2, DSW.2
HD621, Pair of Revivers, AC12