97 strange warm idle

Hi, 97 Truck has 48000 miles. Fuel filter changed 3k miles ago. Problem is slight rough idle, shuddering in warmer weather. Truck has excellent pickup, no hesitation at all. We just got bitter cold weather and engine runs completely smooth. Also when running rough, lights (outside & gauges) flicker slightly. I think the shaking engine causes the belt tensioner to shake a little. The colder it is outside, the smoother it runs, & then lights are on normal. I brought this to a mechanic, did a computer check, no codes, no SES light, batterery and alt. are normal. He couldn't find anything. Any ides are appreciated, Thanks.

When's the last time you did a full tuneup?

What did it include?

The only thing I've done was clean inside throttle body. Changed fuel filter a few months ago.. I thought plugs were supposed to go for 100k miles.

Nope, that is crap. Don't believe that.

Change:

Air Filter
Cap & Rotor
Plugs
PCV Filter

Chances are you are just in need of a tuneup.