Rough/uneven idle, acceleration is fine

Hi. For about the past 10,000 miles I have noticed that my engine has had a rough idle (feels like it is missing at times) and the tachometer fluctuates. It was barely noticeable in the beginning, and sometimes it went away, but lately it has been worse and is now constant. When in drive it sort of feels like an engine that is lugging when pulling a heavy load. No Check Engine Light is present, and has recently passed New York emissions. I had the spark plugs changed recently (using OEM-specified plugs) since I figured the old ones were probably at the end of their expected life, but it made no improvement. I sprayed Seafoam into the air intake when doing the last two oil changes, which noticeably smoothed out the idle, but the effects was only temporary. Using fuel injector cleaners and DryGas in the gas tank has done nothing. Still accelerates smoothly and with plenty of power during both light and heavy acceleration, only the idle is the issue. It has not stalled on me (yet). I have also recently changed the air filter and checked the air intake hose, and nothing seems to be loose or disconnected.

Thank you in advance for any suggestions :-)