System Restart Problem

All my PCs are connected to UPS. If I disconnect the UPS from main power supply, it provide supply to the PC from the battery. But when when electricity goes, some of the PCs gets restarts. This happen only when the electricity goes. I have already changed the power supply many times, also changed the position of PC. But the problem still persists.
P4 2.8GHz, Intel original 865GBF Motherboard, CDROM, 80GB HDD, 1.44 MB Floppy Drive, LAN Card.
Thanks in anticipation.