If you turn the key to run but do not start the truck the check engine light (oil, temp, batt, abs, etc too) should come on. If it does then the light is good. If not then replace the bulb.

On a 94 you should turn the ignition key forward to run (not start) and back 3 times, then leave it in the run position and the check engine light will flash the codes.

I theorize it is a vacuum leak. When you are off idle the ECU compensates for the extra air with extra fuel and you will not notice a difference.

When you return to idle the ECU has opens the idle air control valve to set the idle speed and suddenly you are now getting too much air and it dies.

This is my theory and speculation but I would check for a vacuum leak first.

A 94 magnum with 125K probably has a bad pan gasket IMHO. Unless it has already been repaired.