Odd problem with battery when starting car

[mk_re]

Sometimes when I go to start my car everything just blacks out, as if the battery just dies.
Everything turns off , lights, head unit etc. I lose all my h.u presets..

I have found that tapping the battery (ground connection) brings it back to life. Sometimes it takes a couple attempts, eventually the car starts.

What could be causing this problem?

Cheers,
MK.

[wortus]

loose or corroded ground terminal, take off the lead and clean the terminal and the clamp and reattach dont overtighten it and put vasolene on both terminals it stops corrosion.
Then get the battery tested.