Where to look

Hi all
I am at my wits end with my car, it's a 93 Clipper with a 1.8 fuel injected engine, ECU, catalytic converter & carbon canister. To date I have replaced-
Lift pump
Main high pressure pump
Injectors
Injector pipes
ECU
Plugs
Leads
Distributor cap
Rotor arm
The car still struggles to start and runs pretty rough, the Speedo needle jumps and on occasions right around to 120 even when the car is on the drive and stationary, it's definitely not the speedo drive as the car is not moving. I suspect that there must be an electrical fault but where do I start?
Any help would be greatly appreciated.

Replace your major battery to frame and frame to engine ground.

Replace the CTS sensor. (blue)

Have you statically timed your car, easiest way to do it.  Only need a DVOM.

Set all your timing marks to as the pictures show, then follow method 2.

When removing the wires on the Distributor, remove the Plug from the Diz before trying to expose the wires.

The Speedo is Mechanical, you didn't say if it was manual or auto.  But both are Mechanical, and I have seen bad grounds do weird things.

You will want to plump the main connector on the Clocks, and you want to add an additional ground to the cluster
see:
https://www.volkswagenownersclub.com/threads/improving-your-cluster-connections-and-ground.24949/

Thanks Brian, I assume the cts is the coolant temperature sender, why would this make a difference? Not questioning your obvious vast knowledge just keen to know.
Thanks again

The speedo is mechanical as you say hence the confusion, as I have said on starting the engine the speedo will jump all over the place & the rev counter has now decided to shoot all the way around without me even touching the accelerator.
 I have also had the speedo jump all the way to the right whilst stuck in traffic.

Speedo sounds like a bad cable.

CTS the blue sensor goes flakk
Causes cold start hot start and generally crappy running.

Another thing to look at is the residual pressure test out lined in the Bentley.

Fuel pressure regulators go bad, and the residual test will tell you to replace it if it fails.

When you start the car you place the key in the ignition, turn it to run then wait 4 seconds before you engage the starter?

Does the cable sheath provide electrical continuity to the gearbox? If it does, then I guess a poorly-earthed gearbox could become charged and create an electrical induction in the speedo coil

The speedo, and the clutch cable can act as system grounds.  

If you haven't replaced your main battery ground cables Battery to frame and frame to engine I strongly suggest that those are your first things you replace.  

I have heard of both of them being used to start the car and frying a bunch of things.  

Speedo cables can break strands that can get bound by the internal cable sheeth holding on to frayed or broken strands.  

I have also seen them when they break strands that they can cause funny acting speedos as they tend to unwind and bind then when you turn it off they unwind.  

Sounds weird but I have hear it happen.

Some of my better posts are getting rid of all the plastic flanges, and cleaning all your grounds.  They are in the electrical section in my sig.  

Another thing is Vacuum lines are a bane as there aren't that many but a vacuum leak can also be vexing,  

The Air intake tube can hole or get soft where the isv connects to the hose.


https://www.vwvortex.com/threads/i-still-hate-plastic-parts-but-thanks-tolusina.6907318/?id=6907318&highlight=hate+plastic+parts

https://www.volkswagenownersclub.com/threads/digifant-air-intake-tube-repair.32188/

Renewed the main earths yesterday & ordered a new speedo cable, also booked the car in with a fuel injection specialist on Tuesday. Having trouble identifying the cst but I think that I have the right one now.
I sometimes think that because this was one of the last ones made it is a kind of hotchpotch of lots of components but who knows.