Hi mate, are you any further forward?
With regards to your Speedo. I did the wonky idle trick years ago on a 2.0ts
From what I understand the abs speed sensors feed the info to the abs ecu and then direct feed to the main ecu, which in turn sends the signal direct to the Speedo.
So. You can eliminate the abs sensors and abs ecu if you aren't getting wierd sensations under braking and hesitation after pressing the brake. So that just leaves the main ecu or the binacles.
If you have a spare, it is worth swapping to check the Speedo.
What's the condition of the gearbox earth cable? A cheap replacement can be had for a couple of quid from any motor factor.
The reason why I am saying that is that I once took for granted that this earth cable was sound on a 156, it looked fine and secure and I was tearing my hair out trying to find an intermittent signal fault that was effecting all manner of readings... Speedo included. After finally replacing the gearbox earth... The faults disappeared.. I couldn't explain why or how but I can only presume a hell of a lot got through it as an earthing point.
Ian trying to keep this cheap for you mate.. it does sound like an ECU failure but it's too easy to blame just that.
Let me know what you find mate, it is a quandary this one.