slow starting(cranking) problem, told not battery though

So after my car sat in the garage over thanksgiving, I try to start it up on Friday and it slowly started to crank like the battery was dying. It sounded like it was getting stronger as I cranked it and after about 7 seconds it started up. So I go and park it to go to work, the same thing happened. It also happened again the next day (so obviously when the car has been sitting around for a few hours this problem will happen). Last week my I left the lights on when I went to work and my battery died. I had it jumped and it was fine after that, so I figured the battery may be going. So I take the car to get a new battery and they ran a battery test and told me that my battery is still good. I'm just perplexed as to what the problem may be. The guy at the battery shop said he thought it may be another power source slowly draining my battery when it's not running, but I have no idea what it could be from. The lights are off, exterior and interior. My Compustar alarm not even armed when its in my garage so that can't be it. Someone told me that it could be my starter. Any ideas at all?

[JET]

I would check all of your battery connections first. The mitsu starters are really good and rarely go out. It is a possibility though. Is your clutch engaging right off the floor? It is possible that it may be dragging, you should also notice bad shifts.

[A//// Guy]

Did you fully charge the battery after you drained it? The car will charge it some but I would let it charge for a few hours besides. Clean your battery terminals and the posts, ive had that problem before and my car didnt have any power at all, randomly... so I cleaned the posts (oxidation) and the terminals and all was good.