2 bikes, same setup, wattage spikes in 1

I have a Kurt Kinetic dumb trainer with MooFit BT speed and cadence sensors. No problems hooking up to Zwift via iPad. When I use it with an old (circa 1988) road bike I have rehabilitated, everything works as expected - wattage and speed read smoothly without spikes. When I put on my more usual ride - a hybrid with a more relaxed geometry and gears, I get wattage all over the place, and certainly higher than I can achieve on the other bike. Spikes up to 700 watts on a casual ride, then down to 0 or 50 or whatever.

Wheel sizes are correct for the bikes when matched to the trainer in Zwift. I don’t move the set up, so background interference should be unchanged.

Any ideas?