2000 Chevy Silverado Big problems really need help

Ok, I have a 2000 Silverado w/ about 70,000 miles on it. No upgrades have been done to truck and original equipment/parts are always used. I started having problems with my truck a few months ago, it would be hard to start cold or hot, and I was running rich, very rich. When I pressed the gas while driving it would stutter and then catch up with itself and I wouldnt have any problems driving, unless I came to a stop. My gas mileage was horrible. Recently my truck cut off on me in the middle of the road, and once I got it to start again, it was like I was holding my foot on the gas all the way to the floor, but I wasnt. It would sit and idle all on its on at about 5000rpm's. Put it in gear and it would calm down but wouldnt stay running long. Leave it idling high on its own and it would eventually die out. Still, the smell of richness is promenant. The computer codes are nothing but mis-fires, several. And then it want read anything else, like the computer disengages.I need some help, anybody got an answer for me.