Scoreboard, there's some weird "feature" in the earlier Head Units that allows them to adjust the volume as a function of rpms, ambients noise levels, or some such shit. Most people recognize it by the fact that the HU seems to turn itself down sometimes.
I believe there is a workaround. I think if you adjust the "on-volume" to MAX per your manual, the "feature" is disabled.
I have experienced the HU turning the volume down on me. I was wondering if it was some sort of volume limiter or something to prevent me from blowing the speakers. On the freeway with the windows down you need to have quite a bit of volume and after I turned it up, it would go down again. Very irritating.
I will try the MAX on volume technique tonight and report back to let you know if it works.