99 Ford Escort sometimes only running on 3 cylinders

The first time it happened was a year ago, and it chewed up a spark plug pretty bad. My mechanic switched the fuel injectors around and replaced the plugs and it ran fine for almost a year. Recently it started acting up again - I replaced the gas filter yesterday which was very gunked up. Now it idles better and has a lot more power, but a cylinder cut out 3 times on the way to work this morning - my thoughts are there still could be stuff in the gas line from removing the old filter, bad injector, or the coil pack. The plug wires are new as are the plugs. Any other ideas or thoughts would be greatly appreciated!!

Pull the spark plugs out and look at them! If there is impact damage to any spark plug tip, there may be foreign matter in the cylinder. If there is, you could leave that spark plug out, crank the engine (WITH THE HOOD CLOSED!), and try another spark plug. If that one becomes damaged, take the car to a shop that can vacuum the cylinder.

When I recently replaced the plugs, the old ones looked fine - and we did crank the engine with the plug out…