Sub or Amp Problem?

Hey everyone. I have a new problem with my system. I changed amps 5 weeks ago, and it has been sounding really good since. Then all of the sudden, the speakers on my left side went out (not too worried about that, because it has happened before) but my subs started acting funny. The subs will be normal for 10-15 seconds, then they beat super fast like a machine gun, then go back to normal, and then repeat. Any ideas what is causing them to do this? I know basic electrical skills, but get confused on the amps, ohms, etc.

I have 2 Kicker 10" Comp Subs that are wired to a 600W Kenwood kac-729s amp. I'm pretty sure the subs are 4Ohm.