I have heard of a few PT's having sear trouble occasionally and giving the symptoms that you describe.
Often a replacement sear will sort it out if you can see any wear. Although I think like Holly says, I would take the plate off and have a peek inside, it could be just a bit too dry in there or something sticking a bit. My sliding breach was sticking on mine at the weekend and it was ok after I greased the o ring that seals on the barrel.
We try and keep them dry to stop grit being attracted, but then they can be too dry as well.
Kev Todds article on the AAOC forum is very clear as has been mentioned. Kev is a friend of mine and is still into his Pro Targets if you need to contact him for help.
Barry Taylor also is another expert on PT's.