Strange Signal Drop OUT / Technics SH-AC500 Digital Processor

I'm currently using this unit to process the 5.1 dolby from my TV digital coax out to analog stereo. Generally the audio output works properly except for an occasional drop out of the 5.1 digital signal (Dolby digital indicator goes out) and a loss of audio for about a second or two. I finally found that if I flip a light switch on, or the electronic heating thermostat cycles, or the stove burner regulation cycles, the signal light on the processor goes out. I'm thinking about trying the toslink input instead of the coax to see if that makes a difference.
Something is happening at the AC input level to upset the processor and cause it to momentarily drop the Dolby digital signal.
Anyone ever experience anything like this?? I would like to try a line conditioner, but that seems like an expensive experiment.
Any thoughts would be appreciated.