Bad earthing could, theoretically cause this, but there are many possible causes.

Check the following:
1) That the grounding cable (the "middle" one) on the PSU is really connected to earth (I have stumbled across some el-cheapo UPS that break this connection).
2) That the "On" switch is not shorting itself (moisture? rust?).
3) That the PSU itself is not dying or full of crap (moist dust/fur do not make great insulators). Check the stability of the voltage, especially on the +5V ramp and and +5V SB (pin 9 on the motherboard connector, color-coded purple IIRC).

If all of the above fails, you should perhaps contact a medium