As many have said get it checked out. I would add that it would be best to send to Rowland for the check and possible replacements.
I have an ARC D-111 from 1981 purchased from an ARC engineer who was the original. owner. There were only twenty of them made and he checked it out prior to the 1996 sale to me. I've had it checked in the last few years and it still is right on spec. By the way, the amp runs very conservatively, it is massively over-built with 32 output transistors. Even running it hard for hours the heat sinks only get to about 96 degrees (has three cooling fans).
Regards,
barts