Please help, my wife is on edge. Her car has 200K miles and has a few times lost power while belching black smoke and very poor idle. This has happened twice in the last month but now it is running just fine. I replaced the plugs, wires, cap, rotor a few weeks back and last year I installed a new turbo. It seems to me it could be fuel related. What might cause this intermittent severe loss of power and black smoke problem?

My 91 740T does the exact same thing. it is very random so it is hard to troubleshoot. unfortunately i have not found the problem yet. If you find it before me please post it here. Where is the fuel regulator located? I tried the rotor, cap, plugs and wires. That seemed to work...but only for a couple of months so we continue to look. 1 bright spot. With the idle so poor during this condition, I am getting good at driving with 2 feet.

You might want to read back the fault codes (per 700/900 FAQ's on this site) and see if there are any stored that would indicate something like a bad signal from the Air Mass Meter, or the like. If all you get is a 1-1-1 (no faults recorded) then you might want to explore the more mechanical items like a sticking Fuel Pressure Regulator, although you would most likely need to catch it in the act.

Keep in mind that a fault code that indicates a bad or missing signal from a component might just be a bad plug connection, fraid wire etc.

You should or could go ahead and replace the fuel regulator, it's cheap enough to start with. And I had a similar problem as you a year ago on my 780. It turned out for me to be a loose connection at the waste gate solenoid. I would look there and see if its loose or any loose hoses. Other than that I'm no help.
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'88 745T, '89 780T, '93 944T, '98 S70T5