P0125 on a 1996 3.8L Mustang

First of all, I'm driving a 1996 Ford Mustang 3.8L, automatic. New sparkplug wires, CAI, & cat back system...It's got about 135K on it.

I know that the P0125 is Insufficient Coolant Temperature For Closed Loop Fuel Control...

I've already replaced my thermostat and Engine Cool Temp Sensor and flushed my radiator and refilled.. .

Am I crazy to think that my car is running differently, not hotter, but...slower. It's throttle response is horrible especially in the morning when it's cooler. Is this a symptom of what the code I am talking about?

*sigh- If any of y'all have had this problem, how much did it cost you to fix it and what was wrong.. Is this going to cost me hundreds or is it something more simple?

Any ideas?

Please help me.

did you use the proper heat range of thermostat?

What do you mean? I took the old one off, and put the new one in. Sorry, I'm not familiar with specifics...

All thermostats are rated for temperature. You have to get the right temp in the vehicle. For example, most vehicles are 195 deg. thermostats. So make sure it's the right one.
Second, make sure your coolant level is okay. Low coolant can "starve" the sensor and make it think that the temp is colder, but in fact it isn't touching any coolant.
Third, did you install the stat correctly?

#1 is probably the more likely cause, as Comp said.

alot of guys throw a 180 degree thermostat in where there should be 195 because somebody told them the engine might make more power that way! Haven't seen it do anything but put less heat through the vents in winter time, increase emissions, decrease fuel mileage and turn the check engine light on. As well most professional techs will tell you they always use thermostats from the dealer!