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I have had my 650 for about two weeks now and I've only had it freeze once. Ever since it's worked like a charm.

Yesterday, however, something strange was happening. It would take a really long time for it to process requests; it would take 30-60s to turn a page at first, and then it got to the point where it would sometimes take 5 minutes.

If I managed to get to the home screen, there would be no lag (so I could navigate from different directories with no problem), but whenever I would open a pdf, the lag would start. I find this strange because I was working with PDFs that I have been using for the past week.

I have just turned it on since last night and so far I have been able to flip a few pages without overwhelming lag (but still much longer than usual) and even navigating through directories has lagged a little bit.

What could be the problem? I suspect a few things:
1) Perhaps I have too many PDFs uploaded on it; the night before, I did upload a lot more stuff... However, I read quite a lot since then with no problem and I have had more material on here before... That is, I don't think I should consider 100 PDFs totalling 1GB in space to be a lot.
2) I have been highlighting passages for a few PDFs, but I have not been overdoing it. Could something like this possibly underlie the problem?

So, what could be the problem and what possible solutions should I try to fix this?

Thanks in advance.