Intermittent problem with 90 760 turbo

Frustrating and potentially dangerous. From time to time when I travel
on the highway, especially on inclines, my engine stumbles and I lose
power. The last time it happened, yesterday, black smoke starting
coming out of the tailpipe and the turbo guage was in the white zone.
This happened on a long incline, in fairly windy weather, about 38
degrees F, about 75 miles per hour and trying to accelerate to pass
trucks to about the low 80s, at about 3100 rpm. The turbo, whines a bit
loudly, but gives good power usually. I took it to two mechanics but
the car did not do this while they had it. One had it for 2 days about
a week ago when the loss of engine power happened. And today I took it
in and my mechanic said it didn't act up, and only did an oil change
(last one was about 3,000 miles ago). Car does not seem to be consuming
or leaking oil. Every once in a while it will have an erratic idle.
It'll go up to about 1800, down to 400 and back up for a few cycles,
but it hasn't done this lately. No Check Engine light. In November the
car had bad idling problems, reving over 2000 and back down to about
800 rpm, but the check engine light came on. It has an occasional
engine surge issue, but now it is more losing power. The engine is
rebuilt and car had 88,000 miles when I had a rebuilt engine put in.
The car now has 122,500 miles on it. I only use premium gas (93
octane).

Any ideas? Is this car safe to drive?

 

Safe to drive, but the plugs may carbon up on long high speed climbs.

When our '85 did that it took me forever to find out the cylinder head
temperature sender under branch 3 or 4 of the intake manifold was making bad
contact - reseating the connector fixed it for a year or so.

However, that only affected operation with boost. The idle and light
throttle were good until the plugs carboned. Trouble with idle and any
acceleration usually makes me suspect the Air Mass Meter. Often it is the
AMM connector not making good contact, but the AMM itself has a reputation
for intermittents. It usually requires replacement to pin that down, with
wrecking yards being a decent source of replacements.

Mike