this tale reminds me of my 575, that was sitting together with a 576 for a number of years.... in an attic.
I took down the 576 and it behaved like the proverbial old lady... bad HV transformer, recently a bunch of bad transistors...
Eventually I took a look at the 575. Having the habit of first trying out what happens when rising the variac, thermal cam at hand, well... it worked flawlessly!! Did some realignment, not much was needed. It still has all its original caps, bumble bees, selenium stuff and whatnot that might fail one day. Now its sitting in a (dry) basement, on a shelf connected to the power line where I can switch it on from time to time. So fare no issues.
On the other hand, the old lady... the 576 is sitting on my bench now, in regular use, and the step generator begins acting up (again), doing steps only if well warmed up ;-)
Probably the IC that enables/disables the step counter has an issue...