It's the amplifier drawing a lot of current. When you're at low idle/RPM, you're barely trickle charging your battery and AMP is demanding. At higher RPM, you might not notice this. You could get a larger battery and even better, alternator to help ensure you have adequate power. Also, are you direct to the battery? I would typically wire any amp direct to battery with inline fuse so that I was not sucking all that current through any path on my vehicle that could interfere.
It's the amplifier drawing a lot of current. When you're at low idle/RPM, you're barely trickle charging your battery and AMP is demanding. At higher RPM, you might not notice this. You could get a larger battery and even better, alternator to help ensure you have adequate power. Also, are you direct to the battery? I would typically wire any amp direct to battery with inline fuse so that I was not sucking all that current through any path on my vehicle that could interfere.