HELP 2000 sonoma heat problem

[bigg]

ok i have a 2000 gmc sonoma when i first start driving it gets heat like normal after a few miles it starts to blow COLD air without adjusting anything . ive turned it on and off and its still cold . the temp guage reads normal close to 200 like always. after work today i had heat for a few and then COLD????? any help would be great....

 

[1BADAIR]

4.3?
full of coolant?
when it goes cold check and see if the inlet and outlet hoses on the heater core are hot.
could be air, plugged or even a mode door problem too

 

[bigg]

4.3 vortech