I bought 2 1991 Mazak Vtc-41's a couple of months ago and am having quite a time getting them functional.
Thought I'd see if anyone has had any similar experiences and possibly lend a bit of advice.
First of I've spent much time and $ fixing air leaks on the toolchangers. Everytime you fix a bad one you hear another one that wasn't quite as loud.

After that and anchoing , grounding them I still have some disturbing problems. The machine with the least problems keeps giving z axis overloads and after listening to the z axis run I think a teardown is in order. It has a metallic grinding noise going on so this may be where the problem is. A strange part of the servopack (ol) is after I've homed the machine with it just setting there , it'll click and I get the (310) servo off due to z (ol). I noticed that when it happens the Z axis drops about .030" . Any Idea what I'll find ?

The other machine spooks me , as it sometimes seems to function properly (at least as far as I've been able to get) Then it will suddenly rapid in all three axis z-down y-forward and x far left. When it does this it seems the only thing that'll calm it down is to manually move the axis back to the home position. Then it's okay for a bit --- and for no apparent reason rapid again.
It's very disturbing since I can imagine being in the way. I have shut the machine down by the manual . Move it to home push the emergengy off push the off switch and the next time you start it it "may" or may not go on it's wild ride. Any Ideas?

Frustrated

Update . I disconnected the z motor and ran it to establish if my noise was motor or lead screw. Found that oil line to ball screw broken (for some time), motor made no unusual noise. Removed a badly worn lead screw and nut. Time to spend more money.
Was told that the cable going from the z servopack to the NC cpu was only place that all three axis send the encoder signal , so it is suspect. Intend to disconnect and clean connectors and reseat card in motherboard , then try again.

I know that the one that we have had some similar problems, but only in the X axis. We looked in the back where all the cards are on the main board, and noticed that they had become unseated during the move. We took them out and used an eraser to clean the connections, re-seated them and problem was gone. Good luck.

Can't say we've had any problems as bad as yours, but we just bought one of these machines used too. Have been running it a year, and just noticed that the oil line to the z-axis screw was broken. Fixed now, and luckily before we had any problems like yours.

I have seen other machines make crazy unexpected and unpredictable moves when cables or boards weren't connected correctly. Good luck...