Why is my '86 Nissan Maxima overheating I put in a new distributor radiator thermostate. I know the oil is fine and the water movement is ok. What else could be causing this?

Firstly you need to define overheating Is the car just running hot or is it pushing the water out ? Ignition timing too advanced or a lean mixture cause the engine to run hot. But if the system is losing the water then you need to look Further. If the oil level is ok that is fine but check the colour of the oil. If it is milky then you might have water getting into the oil. Remove the oil filler cap and see if that has got a milky residue on it or inside the tappet cover. This could indicate a blown head gasket or worse a crack somewhere in the engine. If you have access to a radiator pressure tester check the system for water leaks, If you are losing water through a leak this could be causing the over heating. Check the water is being drawn from the reservoir, check the level in the radiator as well as in the reservoir. If the radiator cap is faulty it will lose water and also might not draw water back from the reservoir. If you can borrow a pressure tester the remove the spark plugs and pressurise the cooling system and let the car stand for a while. then leaving the spark plugs out turn the engine over with the starter and see if water comes out of the spark plug holes. This would indicate a leak into the cylinders.
If the car is parked and when you start it it runs irracticaly and then clears it could also indicate that water is getting into the cylinders.