When I run a file with a lot of z movements, the Z seems to get lower

Sounds like it is stalling as the z is trying to move up. I would first try putting 3-in-1 oil on the screw and run the z up and down a lot as fast as possible so you can confirm the z is capable again. It could also be the shaft or the lead screw slipping in the coupling. Finally, check the wiring where there may be zip ties or other places where the wires are fastened and look for chafing. This can cause intermittent shorts in the wires.