01 tahoe w/ onstar no sound

put a Pioneer AVIC-N2 in a 2001 Tahoe with Onstar NO Bose. used the PAC OS-1 interface.

PROBLEM: the headunit plays fine until you get between 30-40 volume. the headunit stays on but the sound shuts off. turn the key off and back on and it works fine again.

we called PAC support and they couldnt offer any suggestions except the mute wire which we already had going to the onstar active wire. we explained how everything was wired and they said it was fine, maybe a bad unit (brand new).