Interesting development in my troubleshooting of this problem over the last few days. Many of you may know that the KE-Jetronic CIS-E system on my car includes two throttle switches, one for full throttle enrichment/RPM limitation, and the other an idle switch for deceleration fuel shutoff. The idle switch also provides input to the idle stabilizer valve. Whenever the throttle valve is closed such as when decelerating or coasting, the idle switch is closed (continuity), sending a signal to the computer. If the computer receives this signal AND engine speed is above 1600 RPM, the computer cuts off fuel to the injectors by reversing current to the differential pressure regulator. Fuel flow is then restored by the computer once the engine speed drops below 1300 RPM. This function is intended to improve fuel economy.
Well, it seemed to me that a malfunction of the idle switch might be sending erroneous signals to the control unit, resulting in fuel cut-off at inappropriate times. So I thought I would check out the idle switch (which is buried out-of-sight under the throttle body) to see if anything was amiss. First off, I noticed that the full throttle switch made a very definite clicking sound when engaged. I expected the idle switch to do the same, but it didn't. I then removed the throttle body to get a better look at the idle switch. It was almost completely covered with grease and grime. (Long story, I recently repaired a leak in the intake system at the bypass hose for the air-shrouded injectors. It exits from the bottom of the rubber bellows that connects to the throttle body. Not only was this leak allowing unmetered air to enter the system, but small amounts of oil from crankcase ventilation was exiting in the vicinity of the idle switch. I didn't notice this when I repaired the hose as I removed the bellows and repaired the connection off the car).
I cleaned the idle switch as well I could, and after poking it a bit, the plunger started to make the clicking sound I expected. I checked for continuity when the plunger was in, and it checked out. When I reassembled everything, I left the throttle switches disconnected just to see if the power loss problem recurred. I've driven the car over 250 miles since then, in all kinds of traffic, and the problem has not recurred. This is by far the longest I have gone without the power loss problem since it began many months ago. Not only that, but the car no longer stalls at stop signs. Sure, the idle bounces around a bit since the ISV isn't receiving the proper signal from the idle switch, but the car never comes close to stalling.
I definitely think I'm on to something here. My theory is that the idle switch was malfunctioning by intermittently sending a closed signal to the control unit even when the throttle was open. This behavior was probably caused by a combination of wear (it's the original switch), oil/dirt contamination, and engine vibration. Since the RPMs would be well above 1600 (especially while cruising down the highway, where the problem tended to occur) the erroneous signal would cause the control unit to cut off fuel to the injectors until engine speed dropped below 1300 RPM. This is why the car always returned to a proper idle after I coasted to a stop by the side of the road, or pushed in the clutch to disengage the engine from the drivetrain. If I just let the engine turn with the wheels while frantically pressing the accelerator, power would not return.
I'm going to drive around for a week or two with the throttle switches disengaged just to make sure I've isolated the problem. Once I'm satisfied that the problem is gone, I'll install a new idle switch. I'll provide an update indicating success or failure at a later date. Right now I'm optimistically assuming success.
THANKS TO EVERYONE FOR THEIR THOUGHTFUL CONTRIBUTIONS TO MY TROUBLESHOOTING, ESPECIALLY THOSE THAT SENT DETAILED RESPONSES TO MY EMAIL. YOU DEFINITELY GOT THE WHEELS IN MY HEAD TURNING. I COULDN'T HAVE DONE IT WITHOUT YOU.