89 idle HELP PLEASE!!!!!!!!!!!!!

my truck runs good except when i go to pull out at a stop sign or somthing from idle it drops from 1000rpms to little under 500 thats not good but i dont know what the problem could someone help please
(P.S. it jumps back up to over 1000rpms in a few seconds ):banghead:
last tune up was only about 2 months ago
it has a little over 81 thousand miles on it
whats that "CEL"
and i only get about 14 mpg
and the problem is random when hot and cold sometimes it does and sometimes it doesnt
and thanks for the response pete

my truck runs good except when i go to pull out at a stop sign or somthing from idle it drops from 1000rpms to little under 500 thats not good but i dont know what the problem could someone help please
(P.S. it jumps back up to over 1000rpms in a few seconds ):banghead:

Last tune up? How many miles? CEL present? Fuel mileage? Does it do this when the engine is warm, cold, or both? The problem could be a great magnitude of things, please give more details.

Pete