H&K Warp 7 preamp problems

[GLUGSTER]

I own a Warp 7 212" combo from Hughes & kettner. When I plug into the main input the intermittent volume fluctuations. This is a reported problem in more than one user review from the posters over at Harmony Central, so I'm guessing this issue is not an isolated indecent. I know the problem rests in the preamp voicing section because I mostly use this amp's FX loop return with a modeling pedal to do the voicing, and have no problems with the amp's power section. Any ideas on what would be going wrong with the preamp section to bring these problems on would be greatly appreciated. Thanks again!