I have this same problem and when I inquired about it on this site a year ago, everyone determined it was throttle hang. Does this increase in rpm's happen all the time or do you just get throttle hang occasionally and an increase in rpm's occasionally? Bottom line, if it's not a consistant problem, it's probably your government mandated smog devices having a mind of their own.
If after checking for physical evidence of a throttle problem comes up with zilch, try the throttle hang fix if it really bothers you. I don't care for throttle hang because I can't rely on engine braking all the time and it will probably cause premature brake wear and maybe clutch wear to boot.
Karl
