Service engine light -- on then off, then on, then off

It seems when I drive at 70 MPH for about 30 miles the light goes off. Then when in the city the light will come back on at low speeds. Now that winter has come, the colder it is ( even at slower speeds) the light will go off. When the temp. rises to 35 F the light will come back on. Again with the temp. at 35 F or above and running at 70 MPH the light will go back off. The only other thing that I have noticed is the when the light comes on the running idle ( RPMs) semms higher.

Any help or advise??
Thank you in advance.