1988 Chevy Silverado 5.7 Idle speed control

I have a 1998 Chevy 5.7 I'm having trouble with the idle control while going at a slow speed about 15 to 20 miles a hour the engine does not slow down when you let of the gas. It will keep going at the same speed even when you let off the gas but if you put the transmission it neutral it will immediately go to a idle. Any one else having this problem and maybe know what is causing it?