The Mk5 GT's are getting old now, it could be a number of things (injectors / injector seals / glow plugs / starter motor / old battery / fuel pump / perished rubber fuel lines etc etc) but if you have an error for the G28 sensor then I would suggest you get this replaced first and go from there.
The G28 sensor is pretty cheap but labour to install it will cost a bit as the sensor is located on the flywheel. This should be the correct part for a Mk5 GT with the 125kw diesel engine https://runautoparts.com.au/bosch-cr...sor-0986280421
Also refer this thread: 125kw TDI Golf MkV BMN - Engine Speed Sensor (G2 and P3138 Intake Manifold Dev Errs
I would recommend you take it to Andrew Robinson Automotive in Derwent Park for him to diagnose.
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