random loss of audio

[gduns - with the Lord]

Kate,
If it is the amp, you will also loose your mics.

If it is your mixer, switch to another 2 channels.

If it isnt the amp,

It could be the Sound card starting to go bad. Possibly overheating.

I usually try by-passing things till i dont have any problems anymore.

try plugging from the audio card directly into the amp, (turn down the amp) and play throught the amp without anything else in the line. eliminate one source for the problem at a time.