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I agree with James that this looks like a power supply issue. Probably a weak power supply which would explain the pulsing (hum?) in the repro stages when it drops out of regulation. I'd be checking that the relevent regulated voltages are within tolerances at all times.

Also as James said, wear in repro heads doesnt normally cause a gradual drop off in output. Most of the head's output remains essentially constant throughout the life of the head but excepting for mechanical tape to head contact issues, the playback high frequency response actually improves with head wear right up to the point where the head gap opens and fails. Then there is a sharp drop off in response and the head has no more useful life.