Thc velocity low

Uncheck "void lock" and test again.

Could you post a backup of your config from the Parameters or Settings tab?

The gcode file you are trying to cut may be helpful too. 

Without knowing anything about how your table is configured, your MIN and MAX limit aren't what I would expect.  I would have expected MIN_LIMIT to be -80 and MAX_LIMIT to be 0, which would change your homing directions as well.  I'm not an expert by any means at looking at a config and seeing what's wrong with it and I'm not in a position to simulate it at the moment.  Phill or Tommy will have to take a peek otherwise.

Probably as stupid question, but are you sure that when the THC commands the torch DOWN, that the torch is moving down, and when it commands it UP, you're sure it's moving up?

I do see that you are on v0.176 and the latest version of plasmac I think was v0.188.  So it's probably worth updating, some important stuff was fixed near the end of the plasmac lifecycle. 

Otherwise, some guys have reported a need to change the P value for the THC from 10 to 20.  So you might give that a go and see if it keeps up better.  

This should do the trick:

linuxcnc.org/docs/2.8/html/plasma/plasma....html#update-plasmac

Kind of poking in the dark here, but here are some things that cant hurt to look at:

Have you confirmed the voltage readings with a known, isolated source, such as a 9v battery?

Have you verified that the axes move the commanded amount? For example if you tell Z to move 25mm have you verified it actually moves 25mm? (same with X and Y)

How angled is the piece of material you are trying to cut? Sometimes if the angle is too steep, the shield can touch the material inadvertently.