VW Touareg too rich at idle, both banks.

This one has had me going for a while - working along with mechanics as well.
Codes are P2188 and P2190 for 'too rich at idle', both banks. It also has a code
P1479 for "brake booster mechanical failure" which comes on if you get on the highway. This motor
only gets activated for a second on start up so I don't think it's related to these codes.
I've added a screen shot of the fuel trims along with engine revs. It is on both banks, when you start it from cold
the fuel trims are bouncing around between -20 and -24 o both banks. The other day I left it running till the ECT was
over 90 degrees Celsius and all I had to do was give the engine a quick rev and fuel trims went to normal positive/negative.
Sometimes it takes a few days before the code returns.
The mechanics have changed the oil separator twice with brand new as part of their tests. They also recently replaced
the crank seal for a bad oil leak.
The vehicle has sat for a while so the engine temperature is at about 40 degrees Celsius at the moment and at idle both banks drop to
-10%. I tried giving the purge valve hose a squeeze but they still drop to -10%.
In the image I've attached from the launch CRP123X you can see how the fuel trims both suddenly go up for a while, for no apparent
reason, then drop to the negatives again. I'm trying to work out what is causing both banks to equally go wrong, and correct again together.
The last test I did was to disconnect the secondary air injection hose, block it off, then try blowing compressed air into it, to see
if that had any effect on the fuel trims but it made no difference.

Today I got around to doing some more tests and trying different live data readings - the fuel trims are changing a 
lot with the purge valve opening which may be the clue I've been looking for. Should the purge valve cause so much
change?