Shut everything down, remove then reinstall every circuit card, power cable and buss cable, makeing sure everything is tight. I've seen PC's lockup and give error codes because of these components being loose.
You said that you've already wiped out the operaing system and reloaded, correct?
What you want to do if you haven't is to reformat the C drive 1st then reload the new XP operating system. Even though XP wipes as it loads, I've personally had better luck with doing a manual reformat 1st then allowing the new operating system load onto a "clean" disk.
Once you've got XP loaded up and working properly, i.e. you've got your internet going and you've downloaded all the latest fix's and updates from microsoft.com then I'd start loading the custom software one at time.
I've worked on PC based voicemail systems since their original introduction on 286 machines runiing MSDOS 2.1 to the newer DELL machines running WIN2000 Server. In between I've worked on machines with other operating systems such OS2, OS2 Warp, WINDOS and other proprietary operating systems and these basic PC machines run 24-7 356 for years on end without haveing to replace a P/S or HD every few years......
I've seen P/S's and HD's go 6 -8 yrs without any problems. Generally keeping power surges and lightning out of them is the key to longevity.
Over time the changes in temps and the build up of heat insulting dust can cause an I/O board to either overheat and breakdown or cause enough movement to allow some insulting material to get inbetween the edge connector and cinch connector. PCI cards seem to have more movement than the older IDE cards and SCSI cards used for tape backups and other accessories seem to be more suseptable to this also.
Generally a P/S either works or it doesn't. The only intermittent problem I've ever seen in power supply performance is when the cooling fan locks up and then the P/S overheats causing output voltage swings. A new fan and the P/S works good as new.
Good luck
Volfandt