Howdy Lightdude,
I would not consider 15 minutes of runtime at maximum output before stepping down normal for the 120P. NovaTac rates the 120P to get 30 minutes at the 120 lumen level using a good quality primary cell. I consistently get 28 minutes +/- when using a Battery Station 900 mAh RCR123 cell.
Did you have the opportunity to test the voltage of the battery before you used it? You might try a factory reset and then put in a new Duracell just for the heck of it. It's odd, but some lights like one brand of battery better than others. At the very least, do a factory reset and clean all the battery contact points before trying another runtime test with a new battery........ and if you suspect heat..... then pop it in a glass of water while doing the test..... that's what I do.