I've started using my Soviet Mikrolab KR580IK80 (a clone of the Hewlett Packard 5036A) again, and I've noticed some behaviour which it used to exhibit but now it seems more frequent. Basically after switching on, it lights up as expected, but the keypad oftentimes does not work immediately, except for "RESET" (СБРОС), which does briefly reset the 8-segment display. After a while the keypad begins to work, and it is possible to navigate addresses, store code and run programs. Then after an unpredictable amount of time the keypad stops working again and the machine is unresponsive, until after another period of time (from seconds to minutes), after which it all starts working again. One thing I have noticed is that the contents of memory addresses are not affected by this, that is to say after it unfreezes, any data I had previously entered is still there.
So my question is, why does the microtrainer freeze and then unfreeze like this, and what might I be able to do to reduce how often or how long it freezes? If the answer is not trivial, what could I do to further diagnose the problem?