cold air

I have a 94 s-10. my question is: what would cause my heater to blow cold air after it has been blowing hot air? I am going to change the thermostat and gasket, is there anything else that would cause this?

What is the engine temperature? Is it running cold? If the engine is running cold, chances are its your thermostat. If your engine is warming up to proper temperature, yet your cold, you should be looking at other sources.

it is running at normal driving temperatures. what else could it be?

i have changed the thermostat, will post again if that did not solve my problem.

Check coolant level, make sure it's full. After the engine is at operating temperature, check both heater hoses where they go into the heater core. They should both be very warm, almost too hot to hold with bare hands. If one is noticeably cooler, the heater core is restricted and needs flushing or replacement.