Since the boot to the flash drive shows the correct info., what I'd suggest is to look at and see if you can save a copy of dmesg (you should be able to type dmesg in a terminal or read it as root from /var/log/dmesg) when booted to the flash drive, and when booted to lmde. You'd be looking for entries related to the processor, similar to mine (which is an AMD rather than Intel), example:
2011-05-12 18:09:09 [ 0.053082] CPU0: AMD Phenom(tm) 8650 Triple-Core Processor stepping 03
Compare the differences and see if anything sticks out as possible modules or drivers that may be related? The 2nd thing to do is look in your /var/log/apt/history.log and see what was installed or upgraded May 10th or 11th, when you started having these issues. Hopefully it wasn't much... I wouldn't go re-installing just yet until you have some idea how to isolate the issue, in case it happens again. And maybe you can roll back an update, though I'm not sure the exact steps if you find something.