help! 2003 denali missing and evap issues

[bige124]

having issues and woundering if any one has had this issue. check engine light came on, ran fine, 2 days later it started stumbling and missing bad. has new plugs, wires and a couple coil packs. the reader said multiple cyl mis fire on 2 5 and 6 (5 6 being the worst) and evap system problems. would a gas cap cause it to run lick crap? evap purge selenoid came up last time to. peope done think there related put im my mind they have to. any help would be great?