At this point, I don't think there is much we can do about your lost XP. Do you happen to remember what exactly you were doing? I can try and replicate the scenario to see if there is some weird random bug going on that needs fixing.
I assume that you have already confirmed that this is not a visual bug? (Gain some levels without it jumping back up to 320+?) As for prevention in the future I usually keep my levels in experience obelisks until I need it.
Hmm well, I will go ahead and close this thread as it seems like a weird one time thing with not much info to go on. If it happens again, reopen this thread and give us any details you can remember.