I had posted a few weeks ago about a cause for the Protégé missing, stumbling, and surging while stopped (sticking fuel injector). Since then I have worked on another Protégé and found the other cause.
The plug wires.
A defective plug wire will increase resistance as it heats up which is why the problems start after the car is warmed up and when you turn on lights, ac etc. which draw large amounts of current from the car's electrical system. The problem will also get worse as time goes on and the defective plug wire burns itself up on the inside and lets less power through to the plug from the COP.
Take an Ohm meter and check the resistance in each of the two plug wires. You can do the same for the COPs by setting your Ohm meter to 2000 Ohms and touching each end of the plug wire ( take the wire off first). The resistance should be the same (600-900 Ohms) in both plug wires. If one plug wire is a lot higher than the other, you need to replace both wires. As an example on the particular Mazda protégé I worked on, one plug wire was showing 600 Ohms and the other was showing 1600 Ohms resistance when hot. It is best to check when the wire is hot after driving the car. If you do replace the plug wires, purchase good ones like NGK which are easily twice as heavy and durable as others. Purchased online, the plug wires are about $28.00 for two. Purchased at the auto parts place, they are about $59.00. New plug wires turned this particular Mazda into a "new" car.
Of course it could also be the COP but you can test them the same way and look up the resistance value on the web. Most of the time it is the plug wire.
The sticking fuel injector and bad plug wire are probably the main causes of most of the stumbling etc. problems especially after the car warms up.
So with the fuel injector "cocktail", new plugs, and new plug wires (if needed) your problems with poor idle and performance should be solved.
You will also notice a huge increase in performance and gas mileage if you have a defective plug wire and replace it.
Don't go spending big bucks replacing everything until you have checked out the simple things.

 

Three most common problems i've seen are dying ignition coils, dying MAF & Dying CMP sensor.