1996 Chevy Blazer Continue heating problem

You say that when you hadit flushed the heat worked all right for a while. Had you changed the thermastat? If it is extremly cold where you are then some vehicle do not heat up enough. You can see it on most big truck that cover part of the radiator. Does the temp guage go up to normal operating tempeture. If not then change the thermastat.

One other question had the hoses been removed when you flushed the heater core. Just thinking of a long shot that could the hoses been reconnected backwards.

Ok with the system flushed and new thermastat and hoses on right. Do me a favor and check how hot the hoses get when it has been running a while. If they don't feel that warm then I would recheck the thermastat. You guy might be right as far as the temp comming out, which could be heater control. But I want to make sure you are getting enough heat going to the heater core. When you move the temp control from hot to cold is there much difference in temp not turning on the a/c In reference to a vacuum leak. There is a product that you spray on the hoses. If it finds a leak the engine rpms drop. I had seen it once some time ago. But I can not remember the name of it. They should be able to give you the name at a auto store.