Carlos,
I would check your Delrin ACME nut to make sure it isn't un-threading from the aluminum body that holds it. I'm assuming this is the style of nut you have, but the machine is used so it could be something else. If it is loose, you should re-tighten the plastic nut all the way into the aluminum body, and tighten the jam nut on the back side. On newer (post 2013ish) versions of the nut there is also a cross-drilled set screw hole as a sort of belt and suspenders to keep the nut from un-threading, and if you have this you should tighten it as well.
The issue could also be electronic in nature. Especially if you have a G540 based system, these are very susceptible to overheating, and often need some sort of auxiliary cooling even if they are not in an enclosure. It could also be PC related, especially if you are running with a direct parallel port connection rather than a motion controller like the Ethernet Smoothstepper.
Either way, call us tomorrow and Cory or I can hep you out.
Ahren
CNCRouterParts