Just something to think about,fuel injectors that drip or are clogged or not opening the correct duration and amount or even not spraying a good patterns will all test good.The test is just testing whether they are working at all ,not how well they are working.Then the computer will compensate up to a point to keep things at the proper levels by opening the other injectors longer and wider,which makes the other cylinders rich and the one with a bad injector lean or goes the other way and makes all of em lean except for the driping injector cyclinder which would be rich and fouling that plug causing the miss at lower rpm till the rpm comes up and cleans the plug.
A good 100,000 mile tune -up trick is to replace the injectors.