Unusual problem with LH 2.2 system

[John Bertram]

I'm wondering if anyone can come some ideas for troubleshooting a problem
that I am having with my '87 Volvo 740 Turbo.  It uses an LH 2.2 system for
fuel and an EZK 117 for ignition.  I am also running a Saab APC knock
sensitive boost pressure control system which is totally independent of the
fuel and ignition systems.  Here's the problem:  When I let my foot off of
the gas under heavy boost (12-14psi) and reduce the boost to say about 3
psi, I get black smoke out the tail pipe for 1-2 seconds.  I don't remember
ever seeing it do this before.  If I lift my foot off the gas all the way,
it doesn't smoke.  The car is equipped from the factory with a turbo bypass
valve which I have tested and does work.  I have also tried a known good
mass airflow sensor just for fun.  I have checked all of the grounds in the
system, and the power supply to all of the components.  I have checked the
NTC (pin checked from the LH ECM plug).  Anyone got any good ideas of what
might be happening?  The O2 sensor seems to be working, but I haven't
hooked it up to a scope to see how fast it's actually cycling (I was
checking it with a slow DMM).  I replaced the fuel pressure regulator a
couple months ago, but haven't gotten a chance to check it again.  The
overall fuel consumption doesn't seem to be affected very much though.  The
smoke is just embarrassing.  There are no noticeable drivablility problems.

Anyone got any ideas?