I have a 2005 Honda Civic 5 speed manual. I have no steady heat in the car. Abit of heat and then cold air starts blowing. This was happening especially when the cold is not moving. Biu it has gor worse. I have had the heater core cleaned%2C replaced the thermostat and checked the coolent level. Still on heat and if it does come it is very little and it does not last for more than a few minutes. My temp. gauge at times keeps going up to hot and it stays there for about 10-20 secs and then comes down to the midway mark. This happens every two or three days.
OK, having your heater core cleaned, or flushed, is good, but there still may not be hot water getting TO it. There is a valve that controls the flow, it's possibly stuck or the linkage to it is not intact. IF you can find that valve, try opening it by hand
It's in line on the heater hoses and looks like