This could be many things because it sounds like there is a dodgy component in your system or connection.
It could be the power supply.
Try taking out the power lead, removing the case and reseating all your cards. Now try again.
It could even be the hard drive playing up.
Try turning it on when cold and booting up using a floppy disk, even an old 98 startup disk will do. This will tell you if the hardware is suspect. If the floppy gives problems too and after reseating the cards problems still exist, reseat the memory. All your data even from the hard drive passes through this.