Sub, mabye amp issue

My car has a 4 channel amp where i run my front speakers and amp though. Lately, the sub cuts in and out (seems like it happens when i hit bumps) I got a car place check connection and they reckon my amp was faulty, so i get a new amp and problem is still not fixed.

Fuses are fine. Connections to amp are fine. Got one of those alitmeters out and it seems like amp is only getting a small amount of power.

Any ideas? Thanks

chech the connections where the the fuse holder is, the fuse may also be mostly blown

check the continuity of your power wire, just do a simple ohms resistance test, thats where i would check, also you might check your ground, you wouldn't believe what a bad ground connection would do.