I've had this happen to me often as well. It always turns out to be an app. I wish I could tell you which one, but I usually just flash a new ROM to my phone when it happens. And as stated already, if you open up the market and go to downloads after re activating, you'll have your download list and just have to redownload everything.
There were a few times when I reflashed that I just let me phone sit for a few hours after reactivating it and it automatically redownloaded all my apps, but it only happened a few times like I said, and I've done this probably 10 times now.
For the task killer issue, if you use advanced task killer with the available widget, then open the program up initially, select what programs to add to your ignore list ( DON'T IGNORE THE TASK KILLER PROGRAM, YOU WANT IT TO CLOSE) and then place a widget on your home screen. This will automatically kill all tasks on a click without have the full program loaded into memory at all times. It will essentially open it self up, kill all tasks, including itself, and close out without you seeing a thing.
The funny thing is, this problem had disappeared for me for almost 3 months and just started again a week ago, so I flashed once again. We'll see how it goes this time.
