It sounds like your Z axis is losing steps on the plunge. When it's pushing into the material, it's trying too hard (or the material's too hard) and so, while the computer assumes it's reached the commanded position, it's actually higher than that. When the tool retracts, it goes to a higher position than it should, so you get the impression it's gradually drifting upwards. To fix that, adjust the acceleration in Motor Tuning - cut it in half, and see if things work better.
If the behavior keeps happening, then it could be slippage of some kind. Either the tool itself is retreating into the collet or the coupler that's pushing the Z axis screw is slipping on its setscrew. Use a marker to check these things - make a mark across the coupler and the motor shaft, the coupler and the screw, and across the tool, just under where it goes into the collet. If things move, the marks will show it.