Fuel pressure problem / injector?

[megger]

Hoping someone can give me some advice on this issue.
1.9 tdi A4 AJM. 2001
Car wont start easily or idle once started. I can start it and keep it running if I clamp the fuel return pipe but it conks out as
soon as I remove clamp. I have replaced the injector wiring loom for a new Audi one. I put some injector cleaner in the
tank and had it running well for two days but the problem has now returned. Also put a 2nd hand newer tandem pump on.

Also get fault code 17678 pd unit injector, which leads me to think a faulty injector No 4? maybe. No fault code for the past
2 days until fault returned.

I would really appreciate some opinions or thoughts on possible causes and repairs/solutions.

Thanks in advance.