Nissan Repair: 1991 Nissan 240 randomly overheats, radiator cap, nissan 240

Question
Ok my car randomly overheats, more often then not anymore.  i can crank the car and before i get a few blocks away it's hot, but i can rev the engine either through high speeds or sitting there and sometimes get the needle to drop back down.  afterwards it's fine until i start the car again... everytime that it overheats i end up having to top the radiator back off, it'll loose about 2 inches of water somewhere, and it's not in the overflow tank.  I dont think it's loosing water before it gets hot though, sometimes it wont overheat and when i check the radiator it is still topped off.  I'm stumped.... besides just replacing the entire cooling system.

Answer
Matthew,

It could be that the thermostat is sticking since it does it only sometimes.  Also, if you have not replaced the radiator cap I would do so.  Next, there might be a clogged radiator that is plugging up just enough to get it to overheat.  If all of these fail I would have the system checked to see that the headgasket is not blown.