Cuts guitar sound mid-sustain?

MACBOOK AIR Processor 1.8 GHz Intel Core i7 With latest OSX 10.7.4.

U use Behringer guitar link usg102 interface. It plays fine except if that if you are not strumming it will often just cut out the guitar sound mid sustain. Not sure how to make it stop doing this. Also even though it is very loud, the sound monitor for the guitar track registers ver low on the green meters. Even the recordings when played back are loud and sound great (other than that cut off when not strumming)

How can I fix this?

Thanks