Try plugging an instrument cable into the FX Send and the FX Return. If that seems to fix the problem, then you have a simple dirty switch in the FX Return jack that is easily cleaned! See the FX Return Jack Fix for Sound & Static Problems wiki page for complete explanations.
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Still cuts in and out with a cable in the FX loop

It would, ideally, come in a 250 watt standalone 115 combo (add 8 ohm cab for 400w) and a head version, along with a lightweight 212 cab!! 
But, unfortunately, every year our hopes are dashed. 

