Had a similar issue, changed plugs, and went with aftermarket coil packs, fixed it for a little bit, then the coil packs started acting a fool, had to pony up some cash to buy OEM coil packs, never had a issue after that.
This happens to me all the time. Dealer took a look at it and replaced the canister, but did not fix the issue. There is a thread out there somewhere that used I believe a Chevy filler hose with a check valve? Let me see if I can find it