Problems with 987 suburban
 

I have an 87 3/4ton 4x4 Suburban. The heating system doesn't seem to be right. It usually will roast you out. But I noticed it is blowing cool air out the vents and won't put heat to the defrost or heater by themselves. I messed with the slider and finally got it to work correctly. Only to have it do the same thing the next day and now work correctly. Any suggestions?