Nutone VX-550 rapidly cycles on/off

This vacuum worked fine for a year. Now it's a few months past the warranty and a strange phenomenon occurs.

When you turn it on (either via an outlet or with the direct switch on the unit) it cycles on and off every second or so. All 5 lights flash in unison with the cycling.

The suction (when it runs) is strong. The bag is installed correctly and almost empty. The front door and lower bag compartment are shut and making a tight seal. I can't find any blockages (input or output). I suspected the bag-full switch. There are two tubes that appear to go up to what looks like a differential pressure switch - I assume this senses a full bag. I don't see any blockages in those tubes.

This is drying me crazy! Any ideas anyone?