99 4runner. Bone stock. 5 speed. I first noticed the stumble when pulling my small fishing boat out of the water. Now it stumbles when the engine warms up. I've replaced plugs because I noticed what sounds similar to a spark knock. When I first started her up and drive, it runs like a raped ape. Once it gets warm, it stumbles when I start out in first gear enough I have to push the clutch in and give it more gas to avoid the stumble. Also, if I start out REALLY slow, it doesn't stumble. It's really noticeable if I'm on an incline. It runs without issue once it gets going. I can hammer the gas and it's responsive. I thought I'd ask before I replace coil packs, fuel pump, seniors, etc. Also, if I shift a little early, say from 2nd to 3rd, and hit the gas, you can still hear the subtle spark knock. If I shift later, it's smooth (so only under a load)

 

My runner recently stumbled under load at specific rpms. It was the coil packs.
Cheap and Minutes to install.
Injectors.....not so much !