no heat
 

last week i changed the intake gaskets in my blazer.Then a week later i drove 300 kms with heat and the next day i had no heat and no heat all weekend,then on my way home i drove 150kms with no heat then stopped shut off my blazer and went to the bathroom and when i got back in i had heat again all the way home.The next moring i got up to go to work and no heat again.This is frustrating me i tried bleeding the system with the nose up high and cap off with no success and also tried bleeding by removing the hose from the fire wall with the truck running.What is going on here i am out of ideas??