I agree with wrench, its very common for head gaskets to do that , also partially restricted or lean injectors as well, the sweet smell of antifreeze from the tailpipe is a good indicator and white smoke, black smoke is usually fuel and blue is oil , also start it if it skips ,shut it off quickly and pull the plugs antifreeze wont burn and the plugs will be wet. egr causes black smoke and rough running due to it hanging open and loosing intake vacuum valve seals can get oil on the plugs and cause a skip, lots of ash fouling when you remove the plugs and look at them, how long since its been completely tuned up? plugs and wires can do wonders somtimes.... good luck