unwanted powerdown & reboot

i made a new pc with socket7 plus board va-501
1.2 gb-conner cfs1275. win98se is completely new and stable. but pc powers down and reboots unwanted. i have set off all power-options.urgent help needed.
BTW: i just changed position of videocard)nowat first after isa(3), or last/forth if counted from left. i have 1 isa-sound at second from left. i removed networkcard(for cable modem)-realtek 8029as. i will now wait 15 min. and remount if pc is ok. we will see.