It’s usually a bad earth. Chuffed you sorted it. Simple mechanics, usually something simple.

Alternatively iirc, it’s a bugger to nip up the collar on the trans. Make sure the threads are clean and it is done up all the way. Remove the trans speedo drive to ensure it’s blue as other colours are different gearing. Then you can inspect...

Potentially the plastic insert in the trans is worn or similar in the back of speedo. As pointed out driving with cluster out will rule out the former.

Could just post whats left of the disco in an envelope.

Would a Vespa pull that?

:D