How do I know the difference?

I have my (purchased new) D2 now for a month and I've had a fair share of bugs and quirks with it.

-The mp3 (including ringtones) don't play consistently. At least once a week, my assigned ringtones would default and my music (mp3s) won't play. The only thing that fixes it is to pull the battery.
-The contact list is slooow to load.
-Youtube videos don't play consistently.
-Camera doesn't work consistently. I take a pic and I clearly see that the pic is taken but the file does not save and the generic landscape icon appears in its place.
-pix in gallery was slooooow to load today. Wasn't an issue till now.

I do have advance task killer running. Would it cause these problems? I have to say though, my D2 was buggy prior to me installing ATK.

Does this sound right? My gut tells me no. Do I have a lemon? I have insurance on this with Verizon.

Help!

TIA

Mary