Whoever replaced the whole board for a temp circuit problem was too lazy to fix it, and "has done took you", in my opinion. There are only a few parts in that circuit, and any tech worth a hoot could deal with it in short order. Replacing the board is the most expensive possible solution for you, and coincidentally the least trouble for the tech.
That's water under the bridge now, so I am just tellin' you...........
What you have is probably a bad connection somewhere, with some oxidation that needs a bit of extra voltage to break it down and make contact. After a while vibration breaks it loose again, and there you are.
Might be as simple as a little used loop jack, so it may be time to try contact cleaner on the jacks, plugging into each one maybe 10 times or so to knock off any corrosion.
If that doesn't do it, may be a solder connection that needs to be touched up..
Yes I USED TO work for Ampeg...but I haven't forgotten everything.