Before you take the drastic route and take your Z in to a shop, check some easy things first:

• Is the radiator clear of debris?
  Leaves, dead animals, sticks, or any general garbage you see on the road can make its way in front of your radiator, impeding air flow. If you have a lot of bugs in the fins of your radiator/condenser take a hose and rinse the fins from the rear.
• Does your thermostat work?
  A thermostat that is stuck shut will rapidly overheat your car regardless of speed or weather.
• Is your overflow tank full?
  If it's not full, there is air in your system. Air has a habit of getting stuck in the engine and impeding coolant flow, causing your engine to overheat. If you stop and restart your engine when it is over heating and it rapidly cools back down then this is likely your problem.
• Overheating at high speeds only?
  This is most likely a clogged cooling system or cooling passages. At higher engine speeds the water isn't in contact with the engine/radiator for long enough time to transfer enough heat to keep your engine cool. Scale and minerals deposited by the water are pretty good insulators. Give your cooling system a thorough flush and if that doesn't fix it you'll probably need a new/cleaned radiator.

To avoid all these nasty problems only takes a minimal amount of preventative maintenance.

• Flush your cooling system regularly.
  Every year, two years, whatever. It's not terribly critical unless you have contaminated your coolant with gas/oil(from a bad gasket or fuel pressure regulator, etc). A good flush every other year should be more than adequate to keep your cooling system in tip top shape.
• Avoid using distilled water!
  I can't remember why, but someone once told me why not to use distilled water in your cooling system and it made perfect sense. Either way you'll be fine if you flush every other year or so, regardless of the water you use.
• Avoid coolant additives.
  The Z cooling system is more than adequate, assuming it is in good working order. Additives are not necessary. If any additives makes your stock engine run cooler then your cooling system isn't up to snuff and you should fix whatever is not working properly rather than cover the problem up with any additives.