I've gotten bad subs from Alpine bad coil first 20 mins. and nothing but trouble from there on out. I will never put Alpine subs in a ride again. You def need to see what ohms each coil is at, I keep a meter in my car just for these reasons. Good luck checked grounds? Might of worked loose and when hits hard vibrates to a bad connection. I had a problem like that once my friend was helping adjust amp and forgot the gain was up way to high.