Outlook 2007 + Gmail + IMAP = missing emails

So, I've been having a lot of trouble with outlook and gmail lately. A number of times I've experienced an email just completely evaporating while I was in the middle of reading it. They'll disappear from my inbox with no trace of them anywhere. It was only very occasional, and therefore tolerable. That is until today, when I went to look for an old email from an old friend who's email address I couldn't remember. Apparently at some point outlook "cleared" everything in my inbox dated prior to 1/2008. No trace anywhere in outlook or in gmail. I also checked my outlook .pst files in \Local Settings\Application Data\Microsoft\Outlook\ without any luck. Does anyone have any idea of what might have happened or where these files could be? I doubt I'll ever get in touch with this person again otherwise. Thanks in advance.