Engine hesitation during acceleration
 

Hey all,

I have a 2007 Mazda3 i 2.0L which has a really annoying problem. I have a video showing it here.

It hesitates whenever I accelerate, no matter what gear, and pretty much regardless of the engine speed. It causes my gas mileage to go to around 20-25 MPG when it was 30-33 before. Of course it makes the car accelerate much slower so my 0-60 is something around 15-20 seconds instead of 9-10 like it should be.

When it started:
When I bought the car in August of 2017, it would do the same thing, but only when around 3300-3500 RPM, and I only noticed it in 3rd gear. Sometime in November, it was much worse, about as bad as it is now. It hasn't changed since then.

Things I've tried:
- Replaced downstream O2 sensor
- Replaced spark plugs with Denso TT Plats
- Replaced MAF sensor
- Fuel injector cleaner

Today when I replaced the spark plugs, I noticed quite a bit of oil on the spark plugs, above the threads, so tomorrow I'm going to replace the valve cover gasket. Since replacing the plugs, it doesn't hesitate all the time, only about 1/2-3/4 the time. I don't think the oil is the main cause of the problem, so I'm asking here still.