Ive been having a similar problem for ages with mine. Ive had it looked at 3 times by the pro's and they couldn't find anything worng. Ive come to the conclusion that its something to do with a faulty ground to the mixer. It seems to sort the problem (temporarily) if i mess around with it but you have to keep an eye on your recording volumes because you might be jamming along and not realise that its happened again.
Might help, let me know if you find enything else that works.