An IR(infrared) tester is a gun lookin mawidge that when you point it at stuff it gives you the temp (ours is plus or minus 5 degrees F) basically you would point it your upper radiatot hose and it wouold tell you the temp.

unless your system is perfectly pressureized (not losing water/pressure) it will not boil at 260 anything above and usually the pressure overcomes the cap, lowering the pressure, and cause thing the whole thing to boil instantly. so... you are either running 16 pounds and are somehow still maintaining that with the leak(doubtful), or you are under 260 and your gauges are off and you o have a leak. have you tried the dye that you put in with the raditator, then run it for a week, then use a black light to spot the dye?? hae you just gone out and bought a new thermostat and cap?? for both it shouldn't cost more than 20 USD. im not sure where you would find the dye, but look around on the internet, maybe they ship to mexico.