My truck's idling up and down when just sitting, the check engine light comes on and goes off. Loss of power and bad gas mileage. What can the problem consists of

Idling that goes up and down and fuel economy issues sounds like an air-fuel mixture issue. Too much air, too much fuel, perhaps both. Why? Can't be sure until testing is performed. Could be an emissions concern rolled in as well; too hard to know without testing the vehicle. There will be clear signs of what's going on when testing is performed. Use this link to find some shops near you that can help with your concern: www.repairpal.com/repair-shops