I suggest you read the listed side-effects and warnings about Darvocet. I don't know what type of pain killer it is, but doubt it would cause premature heart beats of any type. I take that to be what you have described as "prebeats". Many of us suffer from this condition, some located in the atrium (PAC) and some in the ventricle (PVC). The latter are the most noticeable. If they become frequent or troubling it may be that your doctor should prescribe a medication to help reduce them.
I tend to agree with Jerry on this one.....but i think that the Darvocet could definately have a little something to do with it since Darvocet just had a MAJOR recall just two months ago for causing heart rhythm issues......personally i wouldn't take another one until you called your pharmacy and the physician that prescribed them....i would definately look into it my friend.....