Hi Sean,
I suspect your no2 injector is faulty, check the injector solenoid status (group 18) all should be the same (0 at idle i recall) this should indicate the circuit from ECU to injector (if not then it could be the loom so carry out resistance tests although your current measuments already suggest circuit is complete).
We have replaced several of these injectors on the 1.9 for same reason, +/- 2.8mm/s is the max allowed value so anything above registers a code.
There are a couple of things to watch out for when doing the job, the injector must be fitted & tightened square to the cyl head posts (if not they can work loose & damage injector/camshaft), the bolt that holds the injector in place is a stretch bolt & should be replaced, the 1.9tdi also has another couple of bolts that hold the shaft in place that are stretch too. There is a procedure to set the clearance of the injector which is basically to allow a clearance when at max stop (largest point of camshaft lobe) so not to damage injector, where you are supposed to use a dial guage to check camshaft fully down then tighten the adjuster bolt to dead-stop then back off either 1/4 turn or 1/2 turn depending on engine. We generally buy exchange injectors from factors (Bosch) which i am assuming is what is fitted to this car, the later 2.0 TDi have siemens injectors which seem to fail electrically & stop the engine from running (giving a relay code) these do not fail in the same way that the 1.9 ones did, they do however get other issues with seals leaking inside the head & cracks in the head which cause rough running.
Hope this helps, p.s it is a good idea to replace the loom inside the head at the same time as fitting an injector £40-odd quid?
Regards, Philip