Troubleshooting stumble at idle in gear

I got out my multimeter and did the tests in the service manual. You're supposed to use a special test box, but it tells you what to do if all you have is a multimeter.

Disconnect the Electronic Control Unit (ECU) 5-pin connector.
With key on, engine off:
- Cavity 1 in the connector should be within 1 volt of the battery voltage; reads 11.3 while battery is 12.3 (within spec)
- Cavity 2 in the connector should also be within 1 volt of battery; it's 11.2 volts, just 0.1 volt lower than spec
With key off:
- Resistance between Cavity 4 and 5 (this is the distributor pickup coil) should be between 350-550 ohms; it reads 443 ohms
- At distributor connector (where it connects to the coil), either terminal should show an open circuit to ground; they both do (over 20000 ohms)
- ECU connector pin 5 (the pin, not the cavity) should show continuity to a good ground; it does (0 ohms to battery negative terminal).
- distributor reluctor air gap 0.008" (checked yesterday)

The last check is to pull the cable from the distributor cap center and hold it 3/16" from the engine. If you don't see a spark while cranking, replace the ECU. If still no spark, replace the coil. I didn't do this check because I do have a spark while starting the engine.

The last paragraph in the test procedure says if all of these tests do not reveal the problem, then the problem is probably in the control unit or coil.

I also measured the following just for kicks.

Voltages (with key on, engine off)
ignition coil: 5.3V at positive terminal, 2.1V at negative (I read somewhere that it should be more like 8 at the positive terminal)

Resistances (with key off):
Ballast resistor: 1.4 ohms across bottom and about 5 ohms across top (normal I think)
From ECU pin 2 to coil negative: 1.0 ohms (in other words good continuity)
ECU housing to battery negative: 0 ohms (ECU is well grounded)