My usual instinct is to look for bad caps if there is a problem that gets worse over some short period of time. If something is unstable, chances are reasonably good that something is not providing a needed current or voltage, because the devices that store that current/voltage are faulty.
Another possibility is heat buildup. But why would heat build up over the described interval? I've previously described a repair I did to a member's Diamond Memory Lane pedal. The member had 2 of them, ostensibly identical. One was quiet as a mouse, and the other had an annoying whine.
I had several e-mail exchanges with their tech support person, and it wasn't until I mentioned that the whine took about 5 minutes to start occurring, that it triggered his insight. It seems that in an early batch of the pedals, the regulator chips they ordered (from a reliable manufacturer and distributor) had off-spec heat fins, which they innocently installed, and didn't discover the issue until later. The device looks like this and should have a thick heat fin, so that heat can be dissipated. In some instances, the fin may be bolted to a larger heat sink or to a chassis, to keep the device cool. In this pedal, the regulator was free-standing, hence relying solely on the heat fin to keep the device at a suitable temperature. Of course, enclosed in a box, there wasn't much natural cooling, so it took about 5 minutes of power for it to get too hot, and the whining would start as the voltage produced began to drift. As the tech suggested, I replaced the device with a new one that had the proper thick heat fin, and the problem was solved.
Why am I telling you this? The Acoustic amp is from 1973, from what I gather. Back when it was made, I'm pretty sure some thermal compound was applied to the power transistors, so that their heat could be easily dissipated through whatever larger surface they were attached to. That stuff generally stays pasty for a long time, but it's been damn near 50 years since it was applied, and I have no idea what sort of storage conditions it was in. It is quite possible that one or more of the power transistors are overheating because their thermal dissipation is insufficient. The transistors themselves are fine and so is the system implemented for keeping them at a suitable temperature. But they just aren't making decent thermal contact anymore, perhaps because the thermal compound dried up.
Just a hunch.