No heat, plz HELP!!!

2003 impala, my air conditioner works fine, all the setting (from 1 to 5) works fine, BUT my heater does not work, i have no warm air blowning out... JUST cold air for about 1 hour, then i have some warm air (but only if i am driving), while i am stading it's againe COLD... please if you know what do to HELP... Thank You.

are the inlet and outlet heater hoses warm at the heater core, after the egine warms up,,, are they warm ?

check for cold air leaks : cowl, recirculation door, hvac module door

inspect temp door operation

oh yeah is this a 3400 or 3800 ?

my impala is 3.4

3.4