Two things that come immediately to mind are:
1) Turbo - intercooler - intake air hoses. Inspect for holes. Remove each and see if there is "rubber rot". The small one at the exit of the intercooler is most common. Any holes, replace the hose. One symptom of this failure is a loss of power when the turbo would be on boost, and significant black smoke from the exhaust. Plugs will likely be fouled if you keep driving like that.

2) The hall sensor in the distributor is shorting out. All early cars (up to 1989) are subject to this. I had to put a new dist in my 88 last year for a no-start, no spark situation. The plastic insulation degrades around this connection on the bottom of the distributor. You may get some more life out of it with some insulation (ie electrical tape) but sooner or later you'll have to replace the sensor or the whole distributor. This could explain the tach jumping, though I've got no idea why it would go UP. Should go to zero with this failure.

Also, it would be a good idea to get a fuel pressure check to make sure the fuel pressure regulator is working right. Of course, a new part probably costs the same as the test... so maybe you just want to replace it to eliminate one variable.
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Rob Bareiss, New London CT ::: '87 244DL/M47- 221K, 88 744GLE- 202K, 91 244 181K, 88 244GL 145K