POST CODE ERROR

hi,
i have a P4 1.7Ghz rdram 400Mhz 64MB each total 128MB
when it fails to start are even sometimes shutdown restarts itself. it give a post code of 5-1-2-3 Phoenix Bios Ver. A01.
while formatting HDD it hangs & gives error:
standard mode:bad fault in ms-dos extender
fault:000d stack dump:0000 0000 0070
raw fault frame:ec=8500 ip=0b15 cs=0bzf fc=3206 sp=8a40 ss=0b5f

I think it’s a RAM prob not shure already check other hardware.
HELP!!!