heater problems
 

i have a 93 corsica and having problems with getting heat from my defrost or underneath. when its about 20 i'll get heat after running it awhile but not much... but on cold days like below 20 i'll get heat but not much but then if i'm going down the highway i'll get hardly anything.. if i idle i'll start to get some heat. the temp gauge rarely goes about the first mark but in the summer it would only go up about a quarter. the thermostate has about 15000 on it. i've replace the water pump within the last year. i've flushed the heater hoses through the heater core and it flowed smoothly. i don't smell coolant or notice any leaking.. i've also replaced the blower motor. could the coolant temp sensor cause this.. or should i try replace the thermostat again. or could it be the heater core. thanks in advance