Ramdom driving problem

Ok, I have a '03 Cavalier 5 speed and randomly afer dissengaging from a gear to neutral my rpms would come to to about 350RPMS and shoot back to 900RPMs and back down. It does it about 4 to 5 times until it starts to ilde normal. It drives me nuts because it will get to the point were the car will turn off after positioning to neutral after drive. LIke i said its a random problem and it only occurs like once every 6 months. I will also have a high idle issue where the rmps stay at like6600RPMs after highway driving. also an intermitten problem.