Not to sound like a Mr Smarty Pants, if it is tire or pressure related, are you confident in the accuracy of the gage when inflating to 120psi?
In other words, is it possible the gage reads higher than actual pressure.
I dont think so. Im using a Joe Blow Pro pump and its pretty much the only one I have left that is accurate. I have 2 Air Tower 5s that the gauges have both broken on and I also have a Joe Blow Sport that the gauge is broken on. Im reasonably confident that the JBP is right.