[AKoch]

The heat seems to work as I am driving over 40 kms/hr or so and stops when I am at a stop sign.

 

[Mr.Bee]

Fan stays constant? Might be a plug in the heater core. If the fan blows weak, might be some gunk in the blower box.

 

[BroHon]

I achieved some moderate to good results by gently flushing the heater core forwards and backwards.
Then burping the air out of the system, park on a hill with radiator cap at highest point...
Engine fully warmed - Rev/burp/add coolant - repeat. About 20-30 minutes.
Was the cooling system opened up recently or have you noticed any leaks?

 

[Ttavelon]

I had intermittent heater problems for a couple of months. In my case the problem was caused by a sticky thermostat valve. The problem ultimately came to light when the valve stuck shut and the truck overheated and blew off a radiator hose. I changed the thermostat out and changed the radiator fluid and the problem vanished. The heater would not work until the radiator fluid was totally full to its normal level. So, heater malfunction can be caused by sticking thermostats or low radiator fluid. Probably other causes are possible but I would start with these.