Audio Research LS25 MKII Problem

I'm wondering if anyone has any experience with the ARC LS25 MKII. I just bought one used. Soon as I got it I put in new tubes, it run fine for the first day. After about 2 hours today it would mute it self, I would get up and unmute it and again 5 minutes later mute itself. After it did this a few times it would become unresponsive and I'd have to power it off for 10-15 minutes. I turned it off and about an hour turned on again, it run again for about an hour and guess what, muted itself again. So far it hasn't become unresponsive since but obviously something is a miss. I read the manual and it states this.

"RESETTING THE CONTROLS" To avoid discharging static to the LS25 controls, contact another surface (Such as a metal equipment rack) to drain away the charge before touching the LS25. If a static charge should "lock up" the microprocessor, making the front controls inoperable, put the LS25 in mute and power down the system and turn off the LS25 and unplug from the receptacle blah blah blah."

Apparently the LS25 shares the same control system as the REF1 so could share same problem.

Anyone with any experience of this problem or any ideas?

Thanks!