Heater blows cold air at driving speeds....

[ellisjay2]

I checked coolant level in radiator last night after driving home with cold air blowing, coolant seems full close to top of radiator. Normally you would have to wait for coolant to cool down before opening radiator cap. radiator felt warm, so i eased the cap off waiting for some to gush out from pressure, and nothing.....coolant is just sitting there few inches from the top of radiator cap. When sitting still at idle heat blows fairly warm, but at driving speeds merges to cold air. Recently changed water pump, and thermostat, heater core was put in bout 2 years ago, no coolant on passenger side floor, Im thinking thermostat issue, but top hose from engine to radiator is warm when I check it after driving. Will flush system today when I get home and refill.