lack of heat from blower

I have a 1990 Ranger. I have recntly had the cooling system flushed at a "drive-thru" oil change {coolant was really nasty brown-my bad}. Since then, I haven't been able to get heat from my blower, the temp gauge runs up about half way, then falls back to normal operating temp, climbs back, then drops,etc...I've added real coolant/antifreeze, and changed the thermostat. the hoses that lead to the heater core get hot but no heat comes out unless the car is indoors. Any help would be appreciated, as it is cold in the Motor City.

Sounds like you have air in the syatem that needs to be purged. This article is for a different vehicle but the purge process is basically the same. Good luck. Regards-Stevo
http://www.supras.com/sogi/techtips/5_heat_cool/5006_heater_core_trapped_air/