1.8t BAM very intermittent boost

Hi guys - I'll apologise now If I can't do this - the vehicle is a Caddy van with the 1.8t engine in. I've found the most success on this forum so this was my first port.

Good friend of mine is having some serious bother with his Caddy van. It's been fine for a few years and all of a sudden it's been having eratic boost issues. One minute it's boosting fine, the next it's down at 5-6 psi and comes back on when it wants. Sometimes if you knock the ignition off and on, sometimes just randomly.

It's had n75 valve replaced, smoke tested (suspect! as we weren't there to see it), maf sensor (known second hand working one.. again suspect!), turbo has been off to check it's mechanically sound and waste gate opens with some resistance.

I've only just got involved and found the p1297 and p1555 fault codes - could anyone kindly give me some vagcom groups/blocks which I can view and what I should be looking for please?

Thanks