[A4] Mixture Too Lean P0171 Error

[Yee, Phil]

I've been getting the P0171 code a few times a week for about a month now,
along with a some amount of pinging.  I've checked my fuel pump power
(fuses, relay), injector piezo resistance measurement (per Bentley manual),
vacuum leaks, and I still get the message here and there.  I did find a
frayed and cracked vacuum hose that goes from the intake manifold to the BPV
line, replaced it, and it really improved things (which eliminated almost
all of the pinging).  I'm running an APR chip with a 1-cooler range NGK
plugs.  The fuel filter is only a few thousand miles old.  The plugs are
also a few thousand miles old, don't look bad, if a tiny bit on the hot
side.

It drives very well 99% of the time.  I get the error when I'm barely have
my foot on the gas at about 1700-2500 rpm (very light load).  The only time
I notice a truly apparent problem is when it pings sometimes under very high
load in the 5000+ rpm range.

Does anyone have any insights, or suggestions on what to check next?  (I
don't have a vag-com, btw.)

Thanks,
-Phil
00' 1.8TQ
01' 1.8TAvant