Realtek HD Audio Mic Problem

I have a problem when trying to record in with a mic. I seem to get perfect quality sound for a random number of seconds then it cuts out (or goes very quiet). It comes back after a while then cuts out again.

I have tried the mic on another PC and it works fine. The input socket appears to be fine (waggling the mic jack in the port produces no cut out or crackle).

I am using Soundforge to record but have tried Audacity too and exactly the same thing happens.

I have updated the drivers just today and this has also made no difference.

Device Manager shows no problems.

Please can someone help me. I'm tearing my hair out with this one!!!