I have that same problem with my original StageScape.
I have 2 news ones that I use as my live mixer and my studio mixer for playing with our live mix.
Anyway that problem you describe seemed to happen to mine after running sound at a venue with terrible voltage/dirty power.
During that sound job, and ever since, that mixer does that thing where it sounds like a 747 is flying through the dive bar :)
It seems that problem is related to phantom power. Adjusting the level on a phantom powered input causes several other channel's level trim to max out full red.
If you can get away with no phantom power, you may be ok. I never got to troubleshoot it completely, but after 3 more years of using it, I started picking up on the phantom power being the common denominator.