My recently acquired 244 seems to have some issues that get worse as the temperature falls. When I first start it, the engine revs to roughly 1500rmp (I don't have a tach, so this is a guess) then slows to a normal speed after 2-3 seconds. It seems to rev higher and for a longer period the colder the temperature is outside. It never has any trouble starting, but it does seem to miss once in a while while idling when cold. Once the engine is warm, the idle smooths out.

When driving, the engine feels like it has noticeably less power when it's cold, than when warmed up. When I changed the spark plugs recently, the old ones looked like the engine has been running lean. I did fix a vacuum leak that could have been causing this and that seemed to improve the throttle response but not power. With this leak fixed, the motor no longer stumbled when the throttle was applied aggressively.

Is the high reving when first started caused by a faulty coolant temp sensor?

I'm currently running a tank of 91 octane treated with techron through the car. It's only the second tank of gas since the previous owner let it sit for a year and half. Could the lack of power be caused by dirty injectors?

Any other potential causes you guys can think of? Things that have crossed my mind are intake manifold leak, maybe another vacuum leak, bad ECU or AMM? Any advice is appreciated.

Thanks,
Matt