1993 Geo Storm Base 4 Cyl 1.6L
Have heat when idling. When driving lose heat and temp gauge drops. Antifreeze level OK. New thermostat.
Try looking underneath the glove box for plastic boot with foam on either end> It fits between the heater console and the vents. If this has been displaced by a passenger accidentally kicking it, it will let cold air from the outside directly in to the passenger compartment. Re-align the boot and see if this is what your experiencing.