PC able to power on but no display

PC able to power it on. CD rom is working, HDD is working but Monitor no display.
-tried to re-seat RAMs,
-graphic card is on board and had tried insert a PCI graphic card on the board.
-swapped power supply.
-took out cmos battery.
But problem still remain.

-Let the PC rest 20min later, and able to have display but after restart the system problem occurring again.
-suspect on board graphic card faulty.
-after changing the motherboard everything is fine.