i have an issue i need you motor heads to help me with. starts and idles fine.list of symptoms that occur after about 20 minutes of driving.1) drives fine but sounds like its only firing on half of the cylinders at stop lights.2) idle drops from 750 to about 5503)when i shut it off it shoots fluid out the over flow hose. lots of pressure.NOTE- the motor isnt heating up. didnt get over 157 degrees last time.i believe its a bad radiator cap. the way the fluid shoots out of the over flow hose it feels like a pressure thing.