[ksa8907]

You're not the only one, ive lost roughly 25% fe with winter... but i dont suppose it gets quite as cold where you live.

I assume you've checked the obvious dragging brakes, alignment, etc. any chance the timing belt/chain was installed a tooth off? Or perhaps is stretched?

You could be getting poor ignition from a bad coil. My car isn't doing so hot in that department, 1 or 2 coils are intermittently not firing properly but its not setting codes. Still drives ok but slight loss in power and sounds like octane knock around town. Too busy and cold to fix it right now.

The irregular idle could be a dirty throttle body or idle air controller.