Rough Running Under Very Specific Conditions

[Cessnaporsche01]

Hi all,

I've owned a '98 986 for a bit over a year at this point as a daily driver, putting 20K+ miles on it in that time. Other than a water pump bearing, it's been very dependable and well-behaved. It does have a behavior that disturbs me a little, and with winter returning, it's back and rustling my jimmies.

Basically, any time the outside air temperature is cool, say, below 50F, and I have the car at partial throttle, under load, between 2000-2400rpm, the engine shudders and vibrates a ton. Like an old Type 4 trying to idle in the freezing cold level of vibration. Let off the gas, or floor it, put in the clutch, or move out of this rpm range, and it feels perfectly fine. Idle's fine, gets decent fuel economy, doesn't smell or act rich/lean.

I can't find anything online about anything quite like this. Has anyone else experienced it? I'm wondering if it's an anti-knock issue, or maybe just plugs that are in need of replacement, or something like that.