2007 Kia Optima, 2.4L 4-cyl, Auto, 120k mi. With car otherwise running great, I recently replaced the leaking valve cover gasket (symptom: oil leaked onto ex. manifold, smoke from under hood), buttoned everything back up and car ran great on 15 min test drive afterward. Son took car out that evening and within an hour car died in the middle of the road, would crank but not start. Towed to my mechanic who analyzed it and found it had a miss on cylinder. number 2. He replaced coil/plug and was able to get the car started, but still idled rough. Mechanic mentioned that it "could" be a bad throttle position sensor (TPS) but wasn't certain. Since it's an old vehicle with other issues (bad heater core, front end suspension clunk) I'm hesitant to spend a bunch of money throwing parts at the car hoping for a fix.

Current status: Car takes 10+ seconds of cranking to start, and doesn't always start. If it starts, it idles very rough, getting worse as it heats up. When stepping on the gas pedal (no load, not in gear), it hesitates then around 2500 RPM and above, runs great, like a new one. 

My first thought was maybe a vacuum leak since I had to remove the PCV hose and another breather on the cover (PCV hose runs to the intake manifold, the other hose from the valve cover to between the MAF sensor and the throttle body). Both checked out , no cracks or obvious leaks. Also, no cracks seen in the intake line downstream of the MAF. I then tried the propane trick, looking for a smoother idle when introducing propane gas to near these hose connections. No change. 

Codes: No codes currently showing, however..... When the car first died, it threw a P0302 (cylinder two misfire). That was the cylinder which the coil/plug were replaced. Code hasn't returned again, but car hasn't really run very long since it first died. The only other code that the car has (historically) given was a P0340 (camshaft position sensor circuit 1). It's periodically thrown this code for years but the car has always run great (at all rpms) despite it. I've always just cleared it and never had an issue.

If the camshaft was physically stuck in the high rpm setting, I could maybe see it doing what it's doing (hard start, rough low rpm but runs fine at higher rpm). Not sure about the TPS my mechanic mentioned.

Any thoughts here are greatly appreciated. Sorry for the wall of text.

Thanks,

Nick