Try a revision G diverter valve if you're running a D. D is a piston valve, like you mention, and can seep a bit of boost. G is a diaphram, so theoretically holds boost better, but might not last as long. G is probably preferred over the D, overall. The aftermarket GFB DV+ apparently works better than even a rev G, but I couldn't personally recommend one. I'll probably try one at some point, but as it stands my rev G is still coping well with Stage 2 after a good year.
Standard Bosch plugs are fine, or the NGK plugs Pudding mentioned. I've got a new unused set of the NGKs if you want to try them.
I agree that these engines (just like the old 1.8T) can feel temperamental. Some days it feels 'ok', others it's boosting its balls off and lighting up the tyres in 3rd. Mine seems more consistent these days, for whatever reason.
Check the basics - boost leaks, pcv, cam follower, dv, plugs, coils. They can all intermittently affect performance to some degree. You can get a smoke test done for boost leaks. Plugs and coils are relatively cheap to renew. You can use the red top Audi R8 coils (buy them from AKS Tuning), they're cheaper than VW coils but do the same thing.
If you can do a bit of live data logging with VCDS it might offer some clues. There are lots of parameters you can choose to log, but I'd suggest low pressure fuel, high pressure fuel, boost, that kind of thing. Hopefully someone more knowledgeable than me can suggest a specific set of parameters to log for general troubleshooting.
If everything checks out, and you haven't already, take the car out for a good long drive, get it nicely up to temp and give it a bit of stick. There's a fair bit of ECU adaptation that goes on, and sometimes it needs a good cycle. The Italian Tuneup, in other words.
Good luck.
