No Heat 98 F150 4.6

Before I changed the Tstat, I got no heat, after changing T stat I get heat for about 20 seconds then i cools off again, hoses at the heater core are hot, and upper rad. hose is hot, I can turn it off and back on and get heat again for about 20 sec. again, any help would be appreciated.

Before I changed the Tstat, I got no heat, after changing T stat I get heat for about 20 seconds then i cools off again, hoses at the heater core are hot, and upper rad. hose is hot, I can turn it off and back on and get heat again for about 20 sec. again, any help would be appreciated.
You may have air trapped in the cooling system or your coolant level may be low ?