Only overheating on the freeway.

Timing may be to advanced for highway use. Check your thermostat. Some thermostats will stay open when the coolant temp gets to hot with the assumption that nonstop water flow will cool the engine which does just the opposite. Also check what thermostat temp you have. Is it 195,190,185,180....180 is a nice compromise especially where you live. And lastly....do you have a fan shroud? If not you should try to source one and install it.
For leaks in the system let the engine idle at night time for 10 minutes and turn on a black light. The black light will pick up any leaks in the system. Leaks don't have to be in the form of liquid to hit the ground. It may evaporate before it hits the ground. And I know Sacramento is running 100 to 105 degrees for the last few days. Is it happening all the time or just when it gets hot like now. Make sure what /6 Dan said. Is everything new on your cooling system? There isn't much to them but a lot of things that can affect them. A crusty radiator can cause all kinds of problems. You might try replacing the sending unit as it may be giving a false reading. Or installing an aftermarket gauge. Good thing is that cooling systems are relatively cheaper to fix than anything else on the car. Check one thing at a time and keep the engine oil full at all times just in case.