This is a relatively common error on these devices. In many cases, the problem is simply poor solder connections on the regulator or the output AMP IC. If you cansolder and feel confident to work on these, open up the unit and look at the connections for hte output AMP. This should be a large multi-pin IC attached to the heat sink. Also attached to the heat sink should be a few 3-pin transistors (regulators actually), that may have bad connections as well.
The large IC should have RSN314 written on it. Check all the solder connections on any 3-pin component in the general area for rings. If present, resolder the joints.
Let us know the results.
Dan