My ctrl-z stops working after a few hits and I can’t make it start working again until restart (and the whole thing repeats itself)
Editing startuphotkeys with
[EDIT:UNDO,634] // Ctrl+Z
[EDIT:TOOL:UNDO,634] // Ctrl+Z
as I found in other topics doesn’t work.
Would really appreciate some help! 
Check that you are editing the correct file. See here: http://docs.pixologic.com/user-guide/keyboard-shortcuts/defaulthotkeys-file/
What operating system are you using?
Hmm I don’t have that folder in my users directory. The hotkeys file is in my main zbrush directory in zstartup.
I’m using Windows7 x64
Your directory might look like this.
C:\Users\Public\Public Documents\ZBrushData\ZStartup\Hotkeys\
Which version of ZBrush are you using?
Thank you it was there! 
But I don’t have the txt hotkeys file there (that’s why I didn’t find it before)
Should I create/copy the hotkeys txt there?
I have 4R6 version.
Yes, copy the file there and add the following lines at the end.
[EDIT:UNDO,634] // Ctrl+Z
[EDIT:TOOL:UNDO,634] // Ctrl+Z
[EDIT:REDO,890] // Ctrl+Shift+Z
[EDIT:TOOL:REDO,890] // Ctrl+Shift+Z
Seems to be working (although on the first program startup it undid about 30 times and stopped and ctrl shift z started capruring textures of my screen 8I ) but the second time it worked like a charm. So I might be back but for the time being it seems ok
Thank you!