Accuracy is key
Is your cooking thermometer accurate? Checking it is simple if you know the boiling temperature at your altitude. At sea level, water boils at 212°; for every 500 feet of elevation gain, the boiling point drops about 0.9°. At 5,000 feet (Denver), for example, water boils at about 203°.
Immerse the stem of a thermometer that can take the heat (that is, register above 212°) in boiling water and take a reading. If it's off and can't be adjusted ― but you don't want to toss it ― simply compensate by the difference between its reading and the boiling point where you live. For example, if you're at sea level and it reads 205° at boiling, subtract 7°; if the pork needs to be cooked to 150°, remove it from the heat when the thermometer reads 143°.
To check thermometers that don't register at boiling or above, first check one that does, then let the water cool to the range of the others and stick them in. Compare their readings with that of the test thermometer, and calculate the differences.