|"ASUS P5B motherboard"|
That's an older model.
Note that you can also have the following problem on the video card, or on the board inside the power supply.
Open up your case and examine the mboard to see if you have bad capacitors, and/or other findable signs of mboard damage .
What to look for, mboard symptoms, example pictures:
Home page that site
- what the problem is caused by
- he says there are STILL bad capacitors on more recent mboards.
Pictures of blown capacitors, other components, power supplies, Athlon cpu's, etc.:
"...the PC can work well for hours. only when i open a video app it freezes. "
Playing certain types of video causes the graphics chip to use more current, and draws more current from the power supply.
"Geforce 8400 GS"
- if it has a heat sink but no fan, make sure that the heat sink is not clogged with dust and lint - if it is, clean it but DO NOT use a vacuum cleaner to do that.
- If the 8400 GS card has a fan, make sure that the fan and it's heat sink are not clogged with dust and lint - if they are, clean them but DO NOT use a vacuum cleaner to do that - and make sure the fan spins at a reasonable speed, otherwise the graphics chip it cools is likely to overheat.
It's common for the fan to have one or both bearings that are sleeve bearings and for those to deteriorate such that the fan can no longer spin at it's intended speed, or for the fan to not spin at all. (If the fan does not spin at all, it's recommended that yoiu should NOT use the computer at all until you have replaced the fan, to avoid damaging the graphics chip.)
If the fan spins but you hear screeching or rattling noises from that fan, most likely to be noticed after the computer has not been running for many hours and then you boot the computer, the bearings are definitely failing - replace the fan as soon as you can.
The fan can be replaced with an aftermarket fan.
- requires your system has a recommended minimum 300 watt power supply.
What is the (max combined output) wattage capacity of your power supply ?
If it's a lot less than 300 watts (e.g. more than 50 watts less) then your power supply is being overloaded at least part of the time when the graphics card is installed, and that overloading damages the power supply, sooner or later.