Low heat out put

My heater is not putting out like it used to. The temp gauge is about 98 Deg. The coolant level is good. When I stop at a red light the heater blows cool. I can switch from floor to defrost and /or vents but with little heat. If I turn the heat control knob there is little difference from the cold to hot. Any ideas what my problem is? Thanks

Defective thermostat.