So I locked the bootloader, and the reboots stopped. As expected.
Now thinking that I wasn't getting reboots before flashing the HSPA upgrade, I decided I'd try re-unlocking - so I unlocked, reinstalled CWM, and wiped and reflashed a ROM, in this case SlimV11. And so far, one nice fast phone and NO reboots. OK. I'm not on HSPA, but that's not the end of the world... only a month max until I get my Nexus 4 and my g300 goes to the wife anyway, but she'd moan like crazy if she was getting reboots.