P.S.I.cho

ok i pulled 553 wich is the thermactic air deverter, i have a feeling its these cylnoids for emissions on the passenger side, how do i know if one has gone bad, i used a vaccume hose with pressure gauge and they all hold pressure. The car runs fine usually but after i drive it on the highway for 20 or so minutes the check engine light will flash on and car will start bucing and runing bad at steady RPMS car runs fine when rpms very or while at WOT, runs fine at idle too.