iphone 4 when i hang down the call power off
 

iphone 4 (just a little water from dock connektor)
i replaced the dock connektor.

Its working fine,but sometimes (2day/1x) when i hang down one call(put from my ear, the light sensor is clear) power off and i cant turn it on again.

I pick the battery for 5min, and it's working again fine (1-2days).

It is battery problem?

Not warming anything.
I cleaned everything.

Thanks!