1992 Chevy Van little to no heat

What I would do is drain the system, then back flush it. This can be done on the lower hose of the heater. Flush the system until the water runs clear. Then reconnect the heater hose to the heater. Refill with the mixture of anti freeze and water. Check with a antifreeze checker to make sure the freezing range is below -50 degrees. Since you are at a elevation of 9000 feet the it will get much colder out and the airis thinner. After filling the system start the engine and let it run until the thermastat opens. You can tell when it happens as you will be able to feel the top hose get warmer. Watch the temp guage. After the engine has been warmed up move each hose to make sure the system does not get vapor locked. Shut engine off and check the wires going to the temp sending unit to make sure it is clean and tight. That could be the fluciating of the temp guage or it may be the sending unit itself.