I never had problems undoing everytime I clear my history. Problems I do have is sometimes the hot keys don't work, so you got to do it manually. Maybe instead of clicking on cntrl Z, you can go up on the Edit menu, and click on the undo option. I usually have that menu open when cntrl Z doesnt work for me. I'm no wiz or anything so I don't know why it does that. If someone knows, I'd like to know.