I put a Tantrum 600.4 in the wifes car about two weeks ago I kept the stock head unit and used pac sni35's for signal.
Now the fronts (RSD 5 's) are playing at real low volume at times
when this happens if I crank the radio volume almost full up the speakers will play fine for a while then one or the other or both will do the same thing again. Any ideas?
Do you have another amp that you could temporarily put in its place?
Or try hooking up an MP3 player with a mini->RCA cable, and feed that into the amp, to see if it still happens. If so, the amp is probably at fault. If not, it's further up the line.