Sounds like it's motorboating. If the power suply filter capacitors have become weak, this can happen. Basically what happens is the power supply fluxuates enough to cause a feedback loop. When power is first applied, the PS caps charge but the load rapidly depletes them enough to shut down the amp. The caps start to charge again, turning the amp back on but the load quickly discharge the caps and the cycle repeats. After awhile, the leaky electrolytic caps begin to work enough to hold a charge and keep the amp on, but just barely.If that's what's happening, you can measure this with a scope across the power supply capacitors. Might be worth just replacing all the large electrolytic capacitors in the amplifier to see if that fixes the problem. I guess it depends how much the subwoofer is worth to you whether you should attempt the fix or simply replace the whole unit.
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