My adventures with the recently-acquired 1991 740 wagon (non-turbo) continue. B230F, east coast car (regina), etc. I've read through the FAQ and started testing the likely culprits but would much appreciate any advice.

Here are the symptoms:

Starts and runs when the engine is cold no problem.

It starts stalling out when hot after driving for awhile. Seems to only stall out when I come to a stop or when I try to accelerate from a stop. Sometimes during acceleration it will sputter like it is going to stall and then be ok for a little bit. It has been very difficult to emulate this behavior without actually taking it on a drive. I did manage after 30 mins in the driveway to get the thing to stall out, but it was doing fine until then.

If I shut the car off after a short drive, it will restart no problem at all, even if fully warmed up. But once it gets to the point where it will stall out, it will not restart unless I let it cool down a lot (like at least 45 minutes, maybe more). Sometimes after it cools down some it will restart but immediately start sputtering and die when I put it in gear and try to accelerate (even if I do so very gently). Sometimes after it stalls out it restarts after a few tries then runs a little ways and stalls again. The end result is almost always a no-start situation and a 45 minute wait to get it started again.

Other (relevant?) info:

When it us running, it runs a little throaty and sounds a little rougher than it should, especially under acceleration. There is a little bit of a knock to the engine under normal idle. Slight shuddering sometimes.

Gas mileage has been very good in freeway driving.

Exhaust smoke is white (in cold weather) or clear (on better days), basically seems healthy. On very cold days or mornings it tends to have more than the normal car's exhaust output, but nothing that look to be of concern.

Idle sits ~600-700 or so, with minor fluctuations, but no major swings.

When I take my foot off of the brake, at idle, there is a very small surge in idle (~200 rpm?) for about a second or two then it returns to normal.

No significant loss or use of fluids. Oil and coolant look clean.

I have tested the RSR and the FIR (swapped RSR with fan relay, no effect and FIR is clicking and I can hear the fuel pump engaging).

The problem seems to be getting progressively worse.

Could this be an RPM Sensor or faulty IAC? Fuel pressure regulator or the temperature sensor?