Need help finding a joke

I've heard this joke before but I don't remember the ending very well. Can somebody please help me out?

A guy goes to a doctor and says "Doc I got a problem, but I'm embarrassed and I don't know what to do?"

Doctor says "Jim, I've been your doctor for years, just tell me your problem, no judgement, no problem."

Jim says "Well I don't have a problem with sex. Sex is great, but I can't seem to ejaculate."

Doc says "What do you mean?"

JIm says "Well I get it up, and we pump away all normal, but then I get that feeling that I need to cum but I can't finish the job!"

Doctor thinks about this for a while and then brings out a gun. The doctor says "Don't panic. This gun's full of blanks. Take this home with you. Just do everything like normal, and when you start getting that feeling, just pull the trigger and see what happens."

Jim likes the idea, and takes the gun and runs home. Lo and behold his wife is already in bed, naked ready to go.

Jim jumps on top of her bangs her all ways to Friday, and then asks her to finish him with a blowjob.

As he feels like he's ready to cum, he takes out the gun and he shoots the blank.

At which point his wife bites off his penis, and a naked man jumps out of his closet and yells "I'm sorry! I didn't know she was married!"

...

and something else. I'd really like to find the original. Anybody know?