I'm just gonna throw some parts at you that can cause the same running condition on early 2000 fords.
IAC valve controls the idle speed and can cause the symptoms described. I've cleaned a few, without any real improvement.
MAF sensor meters the intake volume and is used for the air / fuel mix. I've never seen a bad one on a car before, I've had over 40 vehicles. It can be diagnosed using a scanner with a live feed function. The value the sensor reads is displayed and can be used to determine sensor failure.
TPS sensor is used to measure the position of the throttle valve, with sends an electrical value used by the PCM and control the air / fuel mixture.
ECT sensor is the coolant sensor which values sent can actually cause a rough running engine, but highly doubtful for your problem.
IAT intake air temperature sensor measures the temp of incoming air, and is also used to regulate the air fuel mixture
EGR valve Exhaust Gas recirculation valve can produce symptoms like yours.
This is all for a ford in 2002, I think the next thing you should do is beg, borrow, steal a scanner that can display live sensor function values. Use the net to get an idea of what values should be displayed normally, and find out if there are any clear ways to determine that a sensor is OOS and is failed or failing.
In the ford with an EGR valve, one of the solutions is to remove the valve and remove the carbon buildup that may be blocking the full travel of the valve sticking at in one position. A stuck or sticky valve usually has tons of carbon buildup inside that can be removed with throttle body cleaner, or my personal favorite brake cleaner.
Good luck! And seriously ask everyone you know if they know anyone with a decent scanner, and call all your local parts stores and ask if they're scanner provides live data monitoring.
When I was stumped a while ago, like 10 years, I purchased an expensive scanner because I could have spent that much easily throwing parts at the car. It worked well and I was able to figure out there were two sensors returning out of spec signals, the TPS and the ECT.
OR, you could always just take it to the dealer, the darkside. I actually would consider the dealer now, you've invested a bunch of time already, and future time spent may be not worth it, depending on how you value your time.
