Interesting Problem

[Chevyman98]

I have a 1998 Cavalier Z24 with about 130,000 miles

If the car sits for 5 minutes or more it starts rather rough then once I release the key it is barely running. Once it even's out every 30-a minute the RPM's drop about 300-400 and the go right back to normal range. Then if I shut it off and try it right away it starts fine but it still drops 300-400 RPM's and goes back to normal range. So all the symptoms are: decrease in gas milage, rough starting and once it evens out drops RPM's then goes back to normal.

I have replaced the fuel filter, new plugs and boots, and a new crank shaft position sensor.

I got the crank shaft position sensor because It never threw a code till this week and I put the code reader on it and it said "Crankshaft Position Sensor circut malfunction"???

Can Anyone Help?