Have you replaced your thermostat yet? If you're overheating replacing that with a genuine Toyota t-stat is the 1st thing you should do. There are other possibilities, but the t-stat is the most common and easiest to remedy issue (arguably).

How long does it take to overheat? If it's slow and gradual it could be an efficiency issue with one or more components. If within the 1st 5-10 minutes it could be something like a trapped air bubble in the system. Are there any leaks? Here's a thread that goes into more detail on troubleshooting: http://www.toyotavantech.com/forum/s...gasket-failure.

Sorry about the title of that thread, I'm not suggesting it's your HG, but most of the info within may be helpful to your situation. Tim