Boost issues

[Fiction1990]

Sometimes I'll boost to 19psi, but most of the time it'll only boost to about 10psi, I have not found any rhyme or reason when it does it. The only code I have is P0299 underboost. I've already tested and swapped with known good N75, boost pressure, wastegate.
What do I need to look for next?

I haven't had a chance to look at vcds and log actual and requested.

[grayjay]

Logs of actual and requested boost, along with wastegate valve duty cycle and intake airflow (g/s) would help narrow down if issue is on ECU/control side or if problem is mechanical. Also try logging the knock sensor signal voltages, could be that ECU is pulling back boost due to something like knock sensor input (bad tank of low octane gas?, clogged intercooler?).
I know on later B8 2.0 Audis with IHI turbo, a P0299 code is pretty common when the wastegate flapper valve wears out prematurely and develops loose play in the swingarm linkage connection and shaft; allowing exhaust pressure to escape through wastegate before onset of boost. If you had a boost leak somewhere in the pressure side pipes or intercooler plumbing, you would probably see high airflow measurements at the AFM (upstream of turbo), without accompanying boost pressure. However, it does not make much sense that a mechanical problem would be intermittent.

[Fiction1990]

I checked the flapper when I swapped the wastegate.. I checked vacuum on my installed one compared to my K03 on the shelf and the installed one would not hold vacuum, so I swapped them.

That's what throws me is the intermittent part.

[Fiction1990]

Can a failing turbo not make any noise, but boost randomly?

Also I found out the other day, if I easily roll on the throttle I can get boost and keep it up, but if I stomp on it I don't get any?

[Fiction1990]

Anyone have any ideas to try?

[TobiqueSlang]

If you're not getting boost at all when you romp on it and you have no vacuum leaks then to me it sounds like a weak waste gate. intermittent would lead me to the N75 but if you're confident both of those things are good then the best I can offer is maybe you have a bad PCV or a check valve in the vacuum system somewhere.

not failed entirely but failing under load.