I'm going to assume you're on Stock Froyo with the latest updates (3.29.651.5)? If so, try copying all of your files off the SD card, reformat the card from the phone, then copy them back. I've seen posts where random reboots seem to be cleared up after doing that.
If it still happens, you might have to try a Factory Reset. Or, if you've already rooted, try reloading a stock or dev ROM.