Try installing the card in a different slot. Use compressed air to thouroughly clean out the inner case and motherboard.
Also, make sure the card is fully seated and the screw is tight.
If that doesn't work, get another wireless card. First, see if the problem comes back with no wireless card installed.
I'd suggest replacing the power supply as well, preferably with a more powerful one.
Somehow, your voltage is unstable or there is not enough power to run everything.