I believe that I answered this question previously. In case this is another, similar question, I'll do so again.
Unfortunately, there's only one reason your engine temp. Would fluctuate to max-min, combined with the heater blowing cold air at idle, and warm air at driving speeds.
You have a partially blown head gasket. What is occurring is exhaust gases are getting past the head gasket and into the coolant system. The air bubbles get trapped inside of the heater core and radiator. At idle speeds, the water pump isn't strong enough to push the coolant past the air bubbles. But as you rev the motor (at driving speeds), the water pump pushes the air bubbles out of the way.
But at sustained highway speeds (60+), there will be so much air in the system, it will over heat, and puke most of the coolant out of the surge tank.
Your only permenant remedy is to have the head gaskets replaced. Block/Radiator sealant will absolutely not work in this case, and will only clog up your heater core, costing you more money in the long run.
You may be tempted to put block/radiator sealer in there as an cheap fix. But I have over 15-years experience in automotive repair, and as a professional, I advise you against this.
As an emergency measure, you can continue to drive your vehicle, but you must watch the temp. Gauge closely. When the needle gets past 210-degrees, or past the half-way mark, shut down your engine immediately. Allow it to cool sufficiently until you can remove the radiator cap, then start the vehicle and squeeze the upper radiator hose several times while the engine is reved up to at least 1500 r.P.M.S.
This will bleed the air out of the system and allow you to drive again until it fills back up with air. Then repeat the same procedure until you can get home.
This is only an emergency measure, and if you continue to allow your engine to run too hot, the engine will seize up and you won't be able to drive it at all.
I hope this helps
Monday, January 4th, 2010 AT 5:35 PM