I know these are silly things to mention, but I noticed how quickly they can cause your temp. to shoot up, and even if they don't help you, they might count for advice...

1 - Check if your overflow hose is properly connected.
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2 - Electric fan's wire polarity.
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3 - Fan belt loose. This is an easy one to miss!
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4 - Antifreeze/water ratio weak (especially in conjuction with a disconnected overflow!)
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5 - I've also noted that a bigger radiator mounted to an unmodfied body also heats up a LOT quicker.

Something you could check into is a product to clean out the corrosion in your water system. Usually a small bottle added to your coolent. This helped me quite a bit by making 2 (almost unnoticible) leaks evident.

Link for some advice/direction if it is the head gasket:
http://datsun1200.com/modules/mediawi ... tle=Changing_Head_Gasket.