I'm assuming this was bought from an individual, otherwise you would have gone back to the used car dealer, correct? Caveat emptor.

We could speculate on what's wrong, and come up with probably 20 or so possible reasons for your problems. However, since you are working with a mechanic right now, you should just continue along that path until it's either been fixed, or you lose confidence in the guy. Be advised that probably the worst thing you ccould do would be to give your mechanic advice based on what anyone on this forum (or anyone else for that matter) has to say.
Hopefully he'll fix it and you will be all set. If not, you can post back again with an updated history.