after driving my recently purchased car for at least 500 miles without any working turbo i was keen to get it sorted. after advise of parts that need replacing ranging from turbo, turbo actuator, n75 valve and mass air flow filter and then a vag-com scan saying i had a fault with exhaust gas recirculation valve (n18) i was so happy to find a huge split in one of the major hoses thinking this could be the culprit. i fixed the hose and voila i had a turbo running in my car for the first time

i took it out for a drive and straight away i noticed huge amounts of black smoke from the exhaust when i put my foot down. not put off and with a big smile on my face as the car had some torque and i could hear the turbo whistle for the first time i continued on.
the car was fine for about 20 minutes and i was getting good response, i tried to push it a bit harder and i lost about 40% power and turbo was very noisy. i think it went into limp mode which i have heard about??? i drove home slowly with really bad idling and loads of smoke.

does anyone think it could be as simple as the n18 exhaust gas recirculation valve that vag-com is picking up faulty?
vag-com says the n18 is open or short to ground does this mean it could be the connection that is providing the n18 rather than the actual unit? can i test this with a meter?
does the black smoke relate to the n18?
anything else i should look at?

my car is 1996 audi a4 1.9tdi 110 bhp, part number for my n18 is - 1H0 906 627

any help is greatly appreciated
thanks