Are they totally random or is there a patten? Any change with regard to Rpm?

Maybe time to go fishing with a second channel on the scope?

Or time to sit back and make a list of systems on the vehicle that use/produce a square wave signal.And then start disconnecting them.

I know both ideas may appear long winded but both must be better than £3000.Only to find out that it is a Shielding issue.

Regards Mark