problem with RPM.. for you smart guys !!

[rps-13]

hi my name is Johann Jordan, well i have a 1993 240SX ka24de dohc. when I woke up in the mornings getting ready to go to the university I start my car and sometimes i have to give it 7 seconds while turning the key for it to start and sometimes it starts great, so here it goes the real deal is when it finishes warming up the car sits at 850-950RPM or sometimes 1000 RPM when i shift and accelerate the car accelerates and when the RPM are going down they stop at 1350-1450 rpm for like 6 seconds and then they come down to 950-1000 rpm same thing when i'm making a stop when i press the clutch they come down to 850 and then they rush to 1450rpm for like 5-7 seconds and then they come down to 950-1000rpm. So what do you think guys should I buy a gallon of gasoline and a box of matches and let it burn or fix it... what could it be?, ohh and my knock sensor is down I dont know if it makes the car to do that.. nahh i dont think so!! HELP!!!!