Hey anto84, while I'm pretty confident that between yourself and more particularly MJMP you could sort out the problem, do you really want to take this journey ? To answer all the questions we are going to ask you as we attempt to trouble shoot the issue, you really need to have the MP1 at hand, plugged in. So you would need to buy it (unless he gives you a return policy or whatever ?). So what seems a cheap MP1 may cost a bit more, at least $50 for a new set of tubes... These kind of intermittent problems can be hard to sort out, hey it may be an easy fix ? You may be better off looking out for a MP1 in good nick with no issues, preferably with new(ish) tubes (and better MDRT and noise mod).
Eg The difference between a Volume drop and gain drop will help determine where in the circuit the problem is.
Not to say that MJMP may be able to narrow it down fairly quickly and give you an idea of what you'd be buying...
Anyway happy to help, just thought I'd ask.
Cheers R