Hi I have a 1998 chevy extended cab with a 5.7 engine. Currently it is rough to start. Will crank for awhile before starting. Once started runs fine besides it isles high around 900-1000 rpms. No matter the temperature if you shut it off it just cranks for awhile again and then starts. Even if you cycle the ignition a few times. Im alittle stumped. Please help. Thanks