my car has begun a high idle about 1200 rpm and sometimes 1400. it may last a week or so like this and them become normal again? could this be throttle body?

 

It could be anything from a faulty throttle position sensor to an idle air valve to a hose. I know that BMWs have issues with all 3. I am not sure of all of the components in the 911's system, but it could be something along those lines...

I DO know that something as little as a different model MAF could cause that problem as well. I threw in a 986 MAF into my turbo and received high idle (around 1100rpm). I know that some tuners use the 986 MAF in the higher-staged cars. You might have a 986 MAF in your car and that could be the problem given that the software can no longer account for the different signal?

Any check engine lights?

 

maybe a throttle body alignment?