Pismo boot and lockup problems

Basic pismo, 500mhz 768 ram..had it forever. runs about 8 hrs a day, shut off at nite.

Last week suddenly shuts down dead. Finally reboots, chimes and makes 4 "hisses" from speaker. Works okay for a day or so, or boots up and freezes..random which one happens. Seems to be better after a PMU reset ( less freezing) PRAM battery in or out makes no difference, date is okay, batt reads 4.5v.

Reseated processor, RAM , Airport card.

Thoughts? I am guessing the 4 "hisses" are telling me there is a boot ROM problem? Who wants to be replaced?

Thanks

Dave k