I'm stuck and need some help.

I'm the original owner of a '94 850 sedan with 220k miles. Having a hard time starting: the starter motor can turn the engine over fine, but (usually when warm but sometimes even when cold) the engine will not fire. It sounds instead like pressure is pumping up in the cylinders for a short while and then the engine will fire once, but not catch. This happens 1-8 times in a row, then it will start just fine and run with no problems: full power, no hesitation.

Further complications:

The Check Engine light is on, but the diagnostic code reader only intermittently responds. On any and all of the A and B sockets, sometimes the LED will flash in response to the 1-second button push, and sometimes it won't.
In all cases, the LED flashes when I push the button, but only occasionally do I get a response from the equipment the diagnostic code reader reads. This is new: previously the diagnostic code reader worked reliably.

With patience I can get codes from the reader. So far, I've seen "engine speed sensor signal missing or faulty". Replaced the sensor that reads the flywheel/flex plate, after cleaning contacts and applying dielectric grease. No change in symptoms. Reseated the engine/tranny controllers in their little box. Symptoms remain unchanged.

Cleaned the battery posts, terminals, and the ground points at body just in front of the battery and at the engine. MEasure 0.003 ohms resistence between negative battery post and engine block, bare body metal, all other ground points I found.

Last bit of complication: the heater fan works only on position 4. I measured the voltage after the load resistor near the fan, and discovered the load resistor had failed. Replaced that, and the fan worked for a couple of weeks but then failed.


So, here are my questions.
1. Floating ground?
2. Wiring harness?
3. (It is about time to replace the timing belt, so:) timing belt?
4. Engine computer?

Thanks for the assistance. I can keep my 240s and 740s working forever with no problems, but these 850s are much harder to diagnose.