- change power supply
- check cd rom for dirt / scratches
- change cd rom drive
- low level format hard disk (mhdd)
- change ide cabel
- use different memory module(s)
- check cpu fan - heat problem of cpu
- problem of graphic card (fan of graphic card)
- check the condensers (around the cpu) for rose spots on the top and/or whether there are blown up tops
If none of above works - try a different mainboard and cpu
Simple - isnt it?
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