1990 200 missing/lean under boost

[David Michael]

It's a long shot, but have you checked the control pressure? Moreover, if
you can safely do so, you may want to figure out a way
to monitor conbtrol pressure under boost.
The control pressure is supposed to be very steady after the car warms up,
but I had a control pressure regulatory go bad on me; it increased the
pressure (leaned out the mixture) by a few PSI randomly. The failure was not
related to boost, but if the control pressure is wrong, nothing will work
properly. Remember, HIGHER pressure means LESS fuel....

Dave