before you wipe everything and reinstall it yourself, i suggest you check out apple support website to see if therte are any update, to my knowlege, i think that certain quick silver's came with os 9.2.1 installed already and then after, did apple release a downloadable 9.2.1 for the rest of the people, so maybe they are different, but it also sounds like a firmware problem, don't ask me what that is, i don't know, all i kno is that i used to have problems with my iMac shutting down without reason and rtandomly from while to whuile and all i know is that after a firmware update it got fixed. so, check for your software update, and then try to reinstall,
but here's another word of wisdom, this came right out an apple support employee's mouth :
"when you get a new system ALWAYS reinstall the operating system from the reinstall, or install cd"
because those sweat shop kids that install the software on every apple computer, are sometimes careless, or the system is outdated, i back to my old imac, i could not connect to the internet through the phoneline modem, then i installed the os from the INSTALL cd, not RESTORE, and it worked... so those may be single isolated cases like yours but they do happen, so that's all i can help ypu with, i don't have a quick sylver, and i installe dmy os myself, on my g4 (see specs below). and i do not have a problem at all.
i hope you fix yours soon, it sounds annoing.