I have a 99' Cadillac Catera Sport. The engine revs up and down and up and down when idle. After ENDLESS hours of GOOGLE....I came across this site. Many others have said its the TPS or IAC, well from everything under MY hood, this car does not appear to have those. So, I am guessing that it has the Electronic Throttle Control system??? Never really heard of that....and cant find ANYTHING on google. Looking at what appears to be the throttle body there is a black rectangle box connected to it. What is this? Is it the electronic throttle control that I hear about? Or would this even be the issue. Plus, only started doing this in colder weather.