Random SES light problem..

I recently bought a 01 silverado from a family member...they bought the truck from a dealership with 15k on it..i got the truck with 90k on it..a few weird things about this truck..it came from the dealership already lowered with 20 aluminum wheels and air intake kit on it..all emblems shaved..its not an ss but very close to it..all it would need is the ss front end..has the shaved in rear roll pan and all..so i'm not sure if this was some special edition or what??
now here is my problem..random SES light..comes on truck starts idleing a lil rough loses some gas milage..when it goes off the truck is idleing smooth and gas mpg goes up...what is causing this?? truck has never had a tune up yet so i'm thinking worn plugs maybe causing a minor misfire?? the rpms do not jump around though..i'm confused as to what it is..truck was mostly drivin around town and maybe once a month was takin on the freeway..other then that its top speed was 35 mph...any advice is helpful..oh and i got that great abs brake light problem as well...

You need to get the codes from the OBD II port. Autozone and Advanced auto will get the codes for free.

cool thanx for the reply..any thoughts on what edition my truck is seeing how it looks like an ss except for the stock front end?? no mouldings or badges anywhere on the truck..has the newer tailights like an ss and rear molded rollpan and dual flowmasters..was this a special order or what?