Stumbling under heavy load?

This sounds like a simple engine misfire; I would run the codes to see which ones are stored. Also, if the engine has not been tuned up, I would start there.

Here are guides to help us get started on fixing the problem:

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

This guide gives you information on the engine misfire:

https://www.2carpros.com/articles/engine-misfires-or-runs-rough

Let's do the tune up and I have included information for your car to help. I would also service the throttle body.

https://www.2carpros.com/articles/how-to-tune-up-a-car-engine

Spark Plugs

NGK: .................... IZFR6K13

DENSO: .................... SKJ20DR-M13

I would install new ignition coils as well, also a very common failure. Check out the images (below). Let us know how it goes.

This means the mixture is rich like a leaking fuel injector, low compression or the exhaust system is leaking before the oxygen sensor.

P0138 - O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)

Here is a guide to help us fix the problem:

https://www.2carpros.com/articles/why-an-automotive-engine-will-run-rich

When you changed the spark plug did you notice one of them being darker than the others?

Opps, that is not good, let's do the compression test to see if the engine is okay. If the compression is good, I would replace the fuel injectors on those cylinders.

Sorry, the engine is hurt and the cylinder head will need to be removed. The compression needs to be at least over 100 psi.

Here is a guide to show you what can cause this:

https://www.2carpros.com/articles/the-reasons-for-low-compression

You can do a leak down test which will show you if the problem is in the valves or piston rings.