Cadence glitch?

I have a stages SB20 and for the last month or so the cadence is all over the place in Zwift (not on the stages app). Basically flips between 180-200 and 50 or so making the resistance crazy, then settles down to 159-170 and normalises over about 39 mi an until it is correct.
Photo attached
New batteries and firmware, and I. Stages app reads correctly.
Any ideas?

Ride on!