2003 Honda Civic Si

132,000 kms. 1.7L Automatic. Vtec.
Car is idling funny. The RPM's will keep bouncing from around 500 to around 2000. I was driving on the highway the other day and my check engine light came on. I took it to my mechanic, and he said it was coming up with an error about my TPS sensor. Now, I know that it doesn't mean that my TPS sensor is bad, it just wasn't functioning right. On my civic, the TPS sensor is riveted to the throttle body. So before I replace the throttle body of 600 bucks, I want to know if there is anything else that could be causing this problem. The needle on my temperature gauge drops down to "C" from the normal midway point when my car is running idle when my blower is on high. Someone told me it might be a coolant temp sensor causing both problems. Does that make any sense? Please help, thanks.