On a 91 300E the sensor is mounted on a small bracket that is suspended just above the harmonic balance on the driver's side. A small tang on the left side of the bracket acts as the timing mark indicator. There should be a wire that travels from one of the harness clusters on the drivers side fender well - under the front of the intake - up over the water pump - and then down to the balance past the belt tensioner shock. The exact path may differ but that should be close. It's somewhat buried down there, but you can see it with the right angle and a flashlight.

Be REAL careful trying tap it or jiggle the wires as you'd be dangerously close to a lot of moving parts while the engine is running.

It's held on with one small nut (8mm?) on a slightly long threaded stud. It's bit of a pricey part (maybe $50-75 ??, I haven't looked in a while). If I strongly suspected it, I'd probably see if I could get one from a junk yard ($5-10) and swap it out.

Some other things you might also want to check are the OVP relay, the air flap in the throttle body (they get gummed up and stick) which should move nearly effortlessly, the idle-air solenoid, the fuel pressure regulator and maybe the ignition coil (in about that order). It doesn't sound like a vacuum issue, which aren't generally intermittent like that.

Good luck, if you find the cause, do follow up and let us know.