1995. Sounds like a dead mother board. If it does not seek the floppy on boot, it usually means just that. You could plug in a speaker (from a old PC) and count the beeps, if any exist. That may also shed some light on what is going on.
2000. Does it try and move the Z axis? Does the Z motor have 2 cables coming from it?
A DAC overflow means the control asked for a move and did not see it. So if the axis did move, then it would be a position loop issue. That is why I asked if it has the 2 cables. The original Z axis had only a armature cable using the Z linear scale for positioning. This could mis-count, thus the machines were modified for a rotary encoder on the motor used in three axis mode. The scale was still used for 2 axis mode.
The DAC underflow means a axis moved without the control expecting it to.