Car Troubles - Idle Speed Variations

Im from cold ol' Minnesota, and I think it may be the problem. Recently, when starting my car [Manual Transmission] its idle speed when in neutral or in clutch my idle speed stays at a constant 2100 rpm. After a bit of driving, it varies from 1200 to 2100, fluxuating from those speeds, going back and fourth. What do I do to fix this besides taking it in, could this cause any problems besides wasting of my precious gas?

Thanks for the help.

I have a friend who just told me he had the same problem on this car. It was simply the idle screw adjustment. It became lose and went out of adjustment. It can be turned by hand. Check it's adjustment