perfect idle after 30 minutes on highway

[scaryc]

my car idles fine, it idles right where it should around 750 rpm. but the engine shakes a lot, im not sure if it's missing or what but when i pop the hood and watch the engine it shakes pretty bad. last night, however, after doing some highway driving probably 30 minutes of it, when i got home and pulled into my driveway i noticed immediately how smooth i was idling. it was perfect. i popped the hood and put my hand on the engine and it wasnt shaking even the slightest bit. it was such a drastic change. today, after doing some city driving, it was rough again. so now i know it has something to do with engine temperature. could it be a sensor? maybe my engine overcools itself?

any help is appreciated

thanks guys

 

[Game Over]

Mine shakes alot too, but I never thought to check after some highway driving. Next time I do a decent amount, I will check.

 

[psychlow]

Check the coolant temperature sensor.

 

[scaryc]

im about to go buy a new coolant temp sensor, if i dont notice any difference, does anybody else have any other ideas what to try?