If the Mountaineer "bucks and chugs" like a stick shift when you forget
to put in the clutch at a stop, THEN it could be a TCC (torque converter clutch) lockup issue. It should set a computer DTC. Go to a quality transmission shop who should be able to confirm the problem for you. If it just dies, with no such dramatics, I suspect a faulty IAC (idle air control) motor or a vacuum leak in the intake system somewhere, causing it to lean out and stall.
That should set a DTC lean mixture code. We always suggest that you don’t replace parts without confirming first that they are faulty. It tends to muddy the waters for further diagnostics and often leads to the kind of situation you have now. Replacing parts on good-running vehicles as preventative maintenance or mileage-related replacement is a good thing, guessing is not, when it comes to repairing drivability problems.
Chino Autotech Inc.
Auto Club SoCal AAR