Hmmm...what do you have the gain set to? Maybe it is clipping when you increase the volume? I have 2 old school 120a2 RF amps, one is mounted into the box facing the back of the fold down seat and the other is mounted into the back of the box facing into the trunk. Both are mounted straight up and neither have ever cut out on me, both are bridged powering a Kenwood 10" like the one you have. My lid is sound proofed so it really gets cookin in there! I would turn the gain down a bit(if set incorrectly) and temporarily mount your amp straight up(vertical) and see if that helps. Good luck.
