Irregular Misfire Mystery
Story Time:
I bought my car from a dealer about 2 years ago with around 90k on it (123k now). During the test drive the car seemed fine, but the day I went to purchase it I was informed they needed to keep the car long because there was a misfire in one of the cylinders. A few hours later the dealership got back to me and said it was a timing issue but was corrected and the code was cleared.
Fast forward to my first oil change (I do them myself), shortly afterwards my car starts to bog down a lot and near stalls out while driving, then the check engine light pops on. I read it using a scanner at the local parts shop and it says there's a misfire in the 4th cylinder (I think 4th). I change out the spark plugs with Autolite junk (foolish I guess) and the problem stops and doesn't kick in again.
A few months later I'm driving a really short distance and the car totally bogs out like I've never had a car. It felt like I was driving on one cylinder, the car would barely drive 10mph and was struggling. I quickly pulled into a parking lot and shut the car off. Check engine light on again. Turn the car back on a few minutes later... it's as if nothings happened. I'll note this is a nice sunny day too and I was driving about 30mph when it did this.
So I go back to the auto parts store (same code as before), get more plugs, replace them all, and drive again. No more issues for a few months. I think this rinses and repeats another 2 more times over the next couple of months. Last misfire was a day after I put new plugs in.
I finally think "Hey maybe this is an issue that's serious and needs a mechanic to look at" . I take it to a well respected shop nearby and they give me the car back two days late with NGK plugs in it, telling me I'm an absolute idiot and to never buy Autolite again. They also wrote a 3 and 4 on the coil packs and switched them so I could later decide if it was a bad coil pack I could easily change that out. I'm charged about $280 for this, which sorta bummed me out.
A while goes by, the car I still think feels weak to drive, and eventually the misfire happens again. And again. And Again. I haven't changed the spark plugs because at this point I'm really not thinking it's them. I'm guessing next it might be fuel injectors, but I'm not sure what my best recourse is for those. Should I change them out with new/remanufactured ones myself? Should I go to a mechanic and get one of those $200 fuel injector cleaner services done? Use Seafoam through my vacuum line? I've had literally no experience with solving this kind of an issue before. On my older cars usually a swap of plugs and/or wires would always fix the issue. I'll note too I've ran a bunch of different gas tank mechanic-in-a-bottle fuel injector cleaners through the car too (Techron, Seafoam, Etc) on an empty tank. I sorta wanna try running Seafoam straight through my vacuum line but I don't know what one to use or how to locate it. Or I guess should I take it back to the dealership and have those mechanics look at it?
I hope someone on here has ran into something like this or has an idea of what my next steps on tracing the issue might be. I'm frustrated and the car runs really great besides the fact it seems to be running a little weak and really occasionally has the misfires.