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My 2005 Dodge Neon has 43,000 miles on it.  After driving for about 30 minutes on the highway, the temperature gague suddenly climbs from the middle of the gague into the red.  It starts fluctuating up and down, too, somewhat erratically.  When I come to a stop I hear a heavy gurgling sound coming from the engine compartment, and I can feel the vibration of the gurgling in my foot on the brake pedal.  The overheating is intermittent at this point, but seems to be happening more frequently.  I'd like to get a sense of what this might be before I take it in for repairs or attempt to fix it myself.  Not even sure where to begin the diagnosis.  Coolant level?  Obstruction in the hose or radiator?  Thermostat?  Any ideas for a cause and cure???  

First of all check your coolant level.  The reason the temperature fluctuates is because the coolant is surging since it's not completely full.  It may be other things but I suspect the first thing is low coolant level.  

After the most recent overheating incident, I checked the coolant level in the reservoir.  It was at the "full-hot" level.  This would suggest the right coolant level, no?  Given the overheating, though, perhaps this is not revealing.  I'm tempted to just add more coolant but I don't want to over fill it.  Thoughts?  

There are a series of steps to check.  First check it cold since I've found that checking when hot doesn't always give me a good reading. Overfilling won't hurt, it will just boil out if it's too much.  Is the mixture of antifreeze about 50/50? If these are Ok, then maybe your thermostat is sticking. Since it seems to have been alright until now that might be the answer.  Also check the oil see if is higher than normal or has a white film on it and notice if any unusual white smoke is coming from the exhaust.  I certainly hope not ( more major problems)