So with it back to fully assembled, check each output channel (before the relay) with your scope for DC offset. Connect the scope with power off. set V/div around 5V. Flip on the power, record your findings for the first channel. Now, do the same for the other channel. With any luck, you have at least isolated the problem to either the LEFT, or RIGHT channel power amp. If the readings are identical, I would suspect possibly the protection ckt, and related components itself. A poorly filtered/weak power supply on the prot. ckt. can also cause this problem.