For what it's worth and anyone with a similar problem, I fixed the thing!
Turned out to be an intermittent connection between the tacho coil & the interconnection plate in the motor - the one with the rectangular socket that the cables plug into. When I checked the signal the first time I...
This machine is abt 4yrs old, and recently has taken to bursts of full motor speed in the wash cycle, which with the water in, makes the belt squeal & the machine jump about! It's about 1 sec full-power then a 30 sec pause to recover. In the past it'd do this a few times (always anticlock...