a/c to oba help

Ok so I converted my a/c to oba. I have a viair pressure switch for on 90/ off 120 psi. The issue I'm having is when my gauge gets up and hits 120 the safety pop goes. Its set for 125psi. I have no idea as to what would cause this to happen. My tank has a 125psi max and is 5 gallons, hence the 125 pop. Any thoughts would be greatly appriciated. also this is not an adjustable safety valve.

Your gauge, or your switch, or your popoff could be inaccurate. I'd put my pennies on the gauge. Gauges suck as far as repeatability, and they don't like to be bounced around much. I calibrate some gauges at work, and more often than not, we have to pitch the gauge and get a new one, or at least with the cheap gauges like most people would have.

I looked on there site and only saw two switches one for 85/105 and the other for 110/150.
I don't know what your using your air for, but more pressure isn't necessarily much more volume. IMO it's not worth it to run any system near the outside recommended tolerances.
Volume of air, either from the pump or the tank is in most cases more relevant.
You can laugh if you want to, but my compressor is set up with double safety valves. My tank is rated at 400 PSI (nominal), one valve pops at 150 and the other at 160. I don't even run a pressure switch, I just dump the excess air. A toggle and a relay to activate the clutch.
I'm a refer tech. and use junk yard refrigeration reservoirs for tanks. I've even used empty CO2, oxygen and Argon tanks.

would having a small leak affect the switch from turning off at the 120 mark? also here is the link for my switch
http://www.polyperformance.com/shop/product.php?productid=1952&cat=567&page=2

i had the same problem with my pressure switch. it was pretty inaccurate (or is it the gauge?) either way, i just bumped up my pop-off valve set point and no more issues. i run a 150/175 pressure switch too and before you ask, i got a good deal on it.