Shaking/vibrating and shutting off

How long since it has been tuned up? What you are describing is most commonly caused by worn spark plugs and wires, but Ford has also had problems with their ignition coil packs.

All gas engines need fuel and air, compression, spark, and all of those things in perfect timing. As a general rule, a fuel problem affects all of the cylinders equally, so the engine will run somewhat smoothly, but with low power. Fuel problems also are usually not intermittent. They occur continuously.

A compression problem can affect one cylinder by itself, or all the cylinders, depending on the cause, but those will also not be intermittent. These are caused by a mechanical problem, meaning something is broken. Those things don't fix themselves while you're driving.

Timing problems are also mechanical and do not fix themselves. If something happens with the timing belt, the engine will never run properly until that is repaired.

That leaves spark as the best suspect, and is in fact what Ford has probably the most problems with. We can rule out the spark plugs since they were just replaced, but the spark plugs wires and the ignition coil pack are equally-good suspects. This is just about impossible to diagnose over a computer. Your mechanic needs to catch it while the problem is occurring so he can see what he can do that affects it. The alternative is to throw random parts at it, then wait and see if the problem acts up again. That is normally the most expensive and least effective way to diagnose this type of problem, and reputable mechanics will resist resorting to this costly method. The better alternative is to leave the car with the mechanic to let him drive it periodically, then work on it as soon as the problem occurs. That is more productive than standing next to a car with a properly-running engine, and waiting for something to happen. You'd be spending money to see nothing being accomplished.