Z axis plunging too deep

First time poster.
I've had my Stepcraft 600 for about 4 years now and every time I go to use it something else goes wrong. When it is something physical, I can seem to figure it out and fix it, but this time it is a programming issue (I suspect anyway) and I really don't know what the problem is.

A couple weeks ago I got a new bit - a 3 wing heavy duty surfacing bit with carbide cutters. I drew up a surfacing code in fusion 360. Made sure to save everything in millimeters (because I've come across that problem before, stepcraft doesn't like inches) and then clicked over to uccnc to run the code. Well, now there's a 1" hole in the bed of my cnc because even though I had set the zero, it went crazy - dashed away from where I wanted it to cut and then plunged really deep.

I thought it was the new program I'd written up. So after walking away from the machine for a couple week (and with a deadline of this weekend for a market I'm trying to prep for) I've come back with fresh eyes. I decided to avoid the new program and the new bit for now, and run a code I know works. Except it has decided to plunge far too deep once again.
This program previously worked just fine. I don't know what the issue is now. the X & Y axis seem to run fine but the Z is plunging way too deep, threatening the bed of the cnc.
It is plunging too deep in the first few lines of the code - it should gradually step down. Again, I've previously used this code with no issues.

Does anyone have any suggestions? I'm running out of patience.