Heat when the fan blow at 2 but cold if the fan is at 3 speed

I have the same problem, as im driving i have some heat but as soon as i stop heat dwindles. my suspicions are like Denis said making sure the heater core is getting good circulation or it could be a dying water pump. This is what else happens: after driving when i shut it off i hear a bubbling (sounds like the heater core has air too but im not 100% sure), also i may have over filled the coolant bucket which may have prevented the bleeding out of the heater core if air is the problem. im too poor and cheep to go to a garage and have it taken care of so im either gonna have to try fixing it my self or bite the bullet on this one. if anyone has any other suggestions please post. (its not the air mixing flap or anything else in the duct work) 04 ford explorer xlt with rear heating

1st make sure the heated core is getting hot, do this by feeling the hoses under the hood that supply hot coolant to the core, if the inlet and outlet are hot then the core has circulation and is not the problem, check this and let me know what you find.