I have a similar problem, but at this point haven't got to the bottom of it. So far have had injection system cleaned, tune up -everything but plug wires, which seem ok, have replaced crank (rpm) sensor, and the coil. Now have found that the temperature guage is dropping back to cold just before it gets to regular running temp (about 3:00 oclock on the 94 850 non turbo) and within a couple of minutes, the stalling, then over revving, up to 3 -3500rpms begins, sometimes ( like this morning)it stayed t about 3500 til I shut it down, other times it goes up and down, tries to stall ,etc. been driving me crazy for about a month. The temp guage, and the fact that I haven't seen the fan running at all, are leading me to having that stuff checked ie, the ECT sensor an associated wiring/ connections, and the fan. Also will probably replace the thermostat while its at the shop.

Not sure this is the answer but I and my mechanic are running out of ideas... let me know if you get anywhere and I'll do the same.