I have a 1998 S10 2.2. When the truck sits at idle, I have heat, but when I drive it the air turns cool. Also when I take the radiator cap off there is no pressure in the radiator . I have replaced the thermostat, clean out the heater core.

Sound like is the water pump and you should test the temperature switch. If you don't have pressure, water pump are bad... Good Luck
I have 2000 s10 http://www.youtube.com/joelin66