Ford Repair: Idle and rev problem, ford mustang gt, throttle position sensor

Question
QUESTION: Hello Mark, I have a 1987 Ford Mustang GT with 210,000 miles. The first couple of times when starting the vehicle with the engine cold it will run for a second or two and stall. After that it will run but with a very rough idle and rev until it reaches normal operating temperature. At that time it will all of sudden start idling ALMOST normal and will run fine above idle. After stopping for gas, shopping, etc... the rough idle and rev will return for about a mile and then all of a sudden smooth out again. It is as if you were turning a light switch on when it runs fine again. I have a scanner and only pulled codes 42 and 92 (both O2 sensors showing rich). The following items have been recently replaced or serviced for this problem: both O2 sensors, fuel pump and filter, fuel pressure regulator, wires and plugs, IAC valve and air passage cleaned, throttle position sensor, map sensor, and distributor stator assembly. Replaced a lot of parts but really didn't worry about it due to the mileage of the vehicle anyway. I consider myself to be a fairly accomplished mechanic but this one has me stumped. Any ideas?
Thanks for your time and service you provide.

ANSWER: vacuum leak-engine coolant temp sensor both of these come to mind-worn keyway on harmonic balancer. good luck.

---------- FOLLOW-UP ----------

QUESTION: Thanks Mark, I am just curious as to how the engine coolant temp sensor could not be working correctly if I am not getting a code for it and my temp guage in the vehicle works fine? Are you saying it could be going bad but not bad enough to throw a code yet? Admittedly, I knew a bad engine coolant temp sensor could cause this problem but didn't give it much thought because of a no code.

Answer
well first the ect sensor is not for your guage you have a different sender for that. if the coolant sensor is sending an incorrect signal but still within specs it will not set code because computer doesn't know vehicle is running a 165 degrees instead of 195 the reading is still within a specified range so computer doesn't know it is wrong therefore won't set code-although it isn't as common as vac leaks and bad iac motors it does happen.