So my 2000 Accent 1.5L Manual (SOHC) with 87000 miles has been having slight idling issue for the last couple weeks. It starts fine and idles fine until I drive it around a bit and let the engine warm up. After about 10 minutes the idle is not as smooth and I get a little bit of engine vibration (like it wants to start stalling) every couple seconds. It's not consistent on reproducing it, though. It seems to me it's directly related to the engine temp. There's no CEL and I definitely have had no changes in fuel efficiency. Acceleration seems pretty normal, too.

Vaccum/intake hoses look to be OK and I don't hear any hissing. PCV valve is fine. Air filter is pretty clean. I've checked the connections to the IAC and the ECT and those are tight. I've put in a new set of plug wires and I think it got a little better, but it may be my imagination. The spark plugs are less than 20k miles old and I had a tune up done 5k miles ago, everything looked fine then. The timing belt and fuel filter were replaced recently, but didnt make a difference.

I'm looking for other things to try. The fuel filter went about 50k without being changed, so I'm thinking I may have filthy injectors. I put in half a can of Seafoam in the tank yesterday, so maybe that will help. This also started right after my last oil change, so maybe I should try a heavier oil weight in the future?

Any ideas? Everything I know about car repair I've learned literally in the last two weeks, but I'm determined to try and fix this myself. Should I inspect the spark plugs? (also, should I get a torque wrench so I can put them back in properly?)

check your cam lobes to see if they are worn out on your cam shaft

I pulled out my spark plugs and one of them looked like part of the insulation had been exploded off of it, so I just went ahead and put in a new set. Things are running smoother now, though I think I'm hypersensitive to what the engine feels like now, so I'm going to sit on it and see if I forget about it. Thanks for the suggestion, though. I'll check the camshaft anyways to make sure everything is in order.