hey guys, i have a problem with my '95 GSR. My cat is making a lot of noise between 3000-4000 and has been for a while. but now when i start it the car hits about 2000 rpm's until it warms up then it goes down to about 500-700 rpm's. it also feels like its missing when i first start it too. (shakes a little bit i dont know if thats actually missing or not). i dont know if this is an immediate problem or if its ok for a while. What sould i do???