turbo and smoke issue yet again

[20vcqdriver]

Not sure if its a coincidence, but ever since refurbing my turbo and fitting charge cooler I have had the engine jumping to limp mode at full throttle with anything over 4000rpm ish showing and the first refurb lasted 100miles before smoking like an old steam engine. Had it refurbed again under warranty and it was still jumping to limp mode but at least managed to pass MOT yesterday first time (emmisions 137HC and 0.56CO). Went to take my son to Chester and it has starting smoking (only slightly so far) again after just 60miles use, this despite the fact I changed the oil to Silkolene 5W40 as they said it could be an oil issue. Could changing to the 5w50 silkolene stop this? Also been suggested that it could be the exhaust as still on the 7A stainless system whilst I save up for the 2.75" version. Getting ECU codes read next week but any help /advice appreciated.
Anyone upgraded to a different turbo and got a serviceable K24 I could have?!!
If this keeps up I'll be getting rid of the heap of junk as patience running out as well as funds...
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