James chin up this is all just part of building your own machine and it's just teething troubles.
It's not uncommon to have little issue's like this and thats why I say to people to give there machines lots of running in time to iron out the bugs before tackling any big or expensive job's.!! . . . . Believe me it could be far worse imagine "striking" out . .Opp's. . mean forking out 5K and have machine that couldn't even do that.

Between us we'll have you sorted and dancing round the garage in no time. .:dance:

Now check everything's tight like you said, whind it up and down by hand feeling for sticky spot's if any major sticks then chances are thats your issue.!. . . BUT . . before doing anything major like striping the Z axis down this is what you could do just to verify it is a sticking Z axis.!. . . . Pack or lift the material higher so the Z axis works in a different place then cut another.

If you want a chat then it's my day off, use the number. . . BUT . . we'll have none of this girly wanting to cry shit round here.!!. . . . Only Yorkie eaters need apply..:lmao: