90 Escort Hard Starting After Warm Up-DTC 16

[frafreg]

Has anyone experienced this error code before? Starts fine when cold. After the car is warmed up and then turned off for about 15 minutes; it is difficult to start unless I push down a little on the accelerator pedal. Once the car starts up again, it runs fine. No other issues. No check engine light. I have about 100,000 miles.

I ran diagnostics and came up with no errors for KOEO or Key On Engine Off.
I came up with an error code of DTC 16 for the KOER or Key On Engine Running.
I am using an Equus 3145 OBD I code reader.
The book that came with the code reader says the following about DTC 16 in KEOR mode:

DTC 16: RPM too low to perform HEGO (HO2S) test
HEGO= Heated Oxygen Sensor
DTC 16: Idle RPM high with ISC retracted
DTC 16: RPM above self-test limit with ISC off

I hooked up a multi meter to the signal lead on the O2 sensor to check and see if there was an obvious problem. The voltage jumped from about 200 milivolts up to about 900 milivolts repeatedly at 2000 RPMs. It varied the same during idle. I know this test isn't conclusive but I think I can rule out the O2 sensor for now.
I also checked for vacuum leaks and didn't find any.

The other possibilities are:
PCV
Fuel Pump Relay
EGR
Valve
Spark Plug Wires
Spark Plugs.

Any help would be appreciated.
Thank you very much.