My 99 blazer now that it is getting cold out only puts out heat part of the time it goes hot/cold/hot/cold any suggestions?been reading on here but only confusing me more thanks for the help

 

sounds like your heater core is slightly plugged, or have air in the system and you need a coolant flush.
if it were air in there though you would hear a "running" water sound from passenger side.

 

Yea...check your coolant levels. If its still doing it, flush your heater core...remove the 2 lines going to it,stick a hose pipe on one end,let er rip!

 

I actually found out something interesting on way home tonight.with the control turned to 82 barely any heat.turn it to 78 freezing cold.i flipped switch several times very rapidly and heat I'm thinking the blend door actuator is partially stripped.i can't hear anything behind dash when you flip the switch back and forth I will know more on Friday my day off so I can check it good thing it has heated seats till then lol

 

blend door could be it...i dont have one on mine, but have seen several threads about it.
the devil is in the details.

 

I flipped the temp control switch back and forth when I started truck this morning about 5 miles out of town had to turn heat off was baking me so really pointing toward under dash to me.i did check coolant this morning it was full and clean looking.i believe had dex cool in it because it is orange

 

That's the problem the actuator the gear must be stripped