Sounds like a Crosman 600, unless (longer odds) it's a 451.
The 600 has a fairly complicated trigger and a bunch of parts that all have to be pretty carefully set up to work really well.The two guys I think you could check with are usernames leonardj, (= Len, from the post above) and IIRC, PABirdman.
Some parts are no longer availble new, particularly the operating cam and the shuttle that runs on it to move the pellet from the magazine to the bore. If these parts are too worn, some less worn used parts must be found, or someone's stash ($$) has to be raided.
Fun pistol, in any case, once running.
Cheers
Trev