Low idle/stalling/power loss

use a multi volt meter and check the reading on your tpi throttle position sensor it could be out of adjustment and also check your idle air solenoid and check the egr valve to make sure its not sticking.most likely one of the 3.you can find the volt setting for the throttle position sensor on you tube as well.hope this helps

Sounds like you have a vacuum leak. My 1997 Ford Thunderbird ran like that until I found a cracked vacuum hose.

How long has this been going on? When a problem like this develops, it's always a good start to check for any fault codes in your engine's computer. Autozone and a few others will do a scan for free and give you any code numbers and technical descriptions, if the computer has stored any faults.
If you've kept track of normal maintenance over time, then a low idle should be a fairly easy problem to fix. That being said, it's not so simple. The computer controls your idle speed under all conditions, and relies on several sensors and switches (inputs and out puts) to work effectively. Have your mechanic check it with the low idle, dying problem in mind. Sounds to me like it needs tweaking-idle control valve, throttle position sensor, air flow meter. See? Get the proper diagnosis, so you don't buy unnecessary parts.
Maybe our ford guru, emissionwiz, will stroll along and find your fix from memory (he's the best).

find your idle sensor tap it with a screwdriver if idle works ok after smacking it replace idle sensor cost for part usually about 50 dollars

Sound like you could have a defective ISC (idle speed control) sensor.

run the fault codes and check for MAF/VAF sensor , accelerator position or throttle position motor problems
check EVAP for venting problems

clean the throttle body

try a fuel filter

the plenium might be cracked, which is pretty common on rangers, or the idle silinoid spring valve is broken