126.96.36.199 is a very old software version for the Pearl.
There are much newer ones available. You can find them stickied at the top of the 8100 forum area. http://www.blackberryforums.com/gene...ilanguage.html
Check your free memory on the device. If it's low, it could explain why you're seeing odd message disappearances.
Options > Status > File Free = ?
No, you can't get all of the old messages delivered to your BlackBerry.
That's not how BlackBerry works.
If you remove your account from BES, and then re-add it... you may be able to get some old messages on your device based on the BES pre-population settings. Maximum would be 14 days or 750 messages... whichever is reached first. How do I adjust message pre-population? - BlackBerryFAQ.com