By checkerboard do you mean variance from light to heavy within the fan pattern? (hope that makes sense) Every gun is the same? Logic would conclude that it is something upstream of your gun. Does your pressure jump around when pulling the trigger? Regulator, wall filters, desiccant systems have been known to do similar to what you are describing when they go bad. (corrosion) What type of piping are you using? Could possibly be a obstruction of some type in the piping. I'm assuming everything was working good at one point so piping diameter shouldn't be the issue but how long are your piping runs and what diameter pipe are you using? Is your paint hose good? Hose could be collapsing internally intermittently. I'd start looking at everything going backwards from your gun. Bound to be something there.