I have a 89 max and I have a problem.
When I drive about 50 mph or more, my thermometer falls down towards the C which cools my heat. In other words, when I drive fast I don't have heat. When I'm idle or driving below 50mph it stabilizes itself between the H and C and my heat works fine. Is this a antifreeze problem or radiator? Or do I need to change my heater core?

 

sounds like you need a new thermostat.

 

I second that. Your thermostat is stuck open.

 

It's a redundant thought, but I experienced the same problems. Rather than have you replace the thermostat, radiator, and heater core as I was instructed to do, the shortest path to the problem is with the water pump itself. It ran fine forever, but the heater never worked at highway speeds. To sum things up, someone before me must have run straight tap water in the cooling system, and the water pump blades looked the the lids off a tin can. If you have anywhere near 60,000 miles or more, and have not done the timing belt, go ahead and save some pennies, buy the water pump, thermostat, timing belt, and tensioner and do the scheduled maintenance (i.e. flushing the cooling system and hose/belt replacements. Be sure and check and make sure you flush out the system really well once you get the pmup replaced too. If water has not been flowing well you will have deposits that will get flushed out of places that they were hiding in and can then cause problems with blackage. Trust me i found out the hard way. There is nothing more fun than replacing a heater core, then have it start leaking becuase you got lazy and didn't want to do that final flush. Darn plastic heater cores. lol