I bought a computer diagnostic reader and I got these codes.
P0440 Evaporative Emission Control System Malfunction
P0141 O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 2)
P1810 TFP Valve Position Switch Circuit (GM)

A problem I am having is that the car could be driving on the highway fine, and then the RPMs will jump to 4-5000 with the engine revving high but no change in the speed at all. It only seems to happen after the car has been driving for awhile, not when first started and on the highway. And not when driving 45 mph or lower in the city.

Add to the fact that it overheated on me today, it didn't get to the red line, but the line just before the red. It seems to be coming from the area where the engine temp sensor is. At least that's where I can hear it hissing. I had checked the overflow and made sure there was water in it a couple of days ago. But I'm wondering if its possible the engine temp sensor is also bad or has a leak around that area to where it didn't give me a reading with the computer?
I believe the fan is kicking on, the normal operating temp is in the middle, 195 deg I think.

I'm trying to learn as much as I can about this car as I've had it about a year and want to save money by doing as much as I can myself. I can't trust a mechanic too much as I've gotten into that 'she's a female let's rip her off' situation before.

Thanks very much!

Correction: its a z-24 2.4 f/i auto