89' 740 Turbo - Engine problem

Bought the car just this last summer to use as a winter beater here in
the Great White North of Canada. It's been running quite fine up until about
a month ago. The symptoms were it started running sluggish, didn't seem like
it was running on all 4 cylinders or not getting enough fuel. It would run
okay in the lower rpm's but once above 2700 - 3000rpm or when you stomped on
the gas it almost seemed like it would cut out and would have black smoke
coming out of the pipe and the turbo gauge would go right up.
I inspected the plugs & wires, which were fine, cleaned up the cap &
rotor contacts and miraculously the thing ran as good as when I got it.
Problem solved I thought , "NOT", now a few days ago the same symptoms
started up again. Checked out the previous fix I did for it and both the
plugs & wires and the cap & rotor were just fine.
Would anybody have any clue as to what I should check for next. Again
this is just a winter beater so I really don't want to spend too much money
on it. Thanks in advance.

Norbert