04 Civic - doesn't blow hot air unless its moving

Just started to use the heater and noticed a problem. This morning I started the car early, because this is the first day we've had frosted windows. 15 minutes later the windows were clear, so I got in to leave for work and the car was freezing. Heater was on, but was blowing cold air. I left for work, and in about 2 minutes it started blowing hot air. I stopped at a stop light and it blew cold, started going again and it changed to hot.

A little help please? My mechanic can't get me in until Thursday...

#1 low on coolant
#2 stuck thermostat
#3 air pocket in cooling system