The preamp section on my series II Carvin R-600 occasionally decides to mute the output or make it really weak and distorted. It usually only happens for a few seconds and then it goes back to normal but still, I don't want this kind of performance. It's got to be some sort of loose connection somewhere but I can't figure out what. Has anyone else had this problem before and/or know what it might be? BTW, I had it fixed a few months ago for doing something similar, except it became like that all the time (unusable) so the repair guy apparently secured a bunch of contacts and it worked perfectly until now.