civic 1.5 engine problem

hi everyone if someone can tell what problem i have in my car
started in a day when it was -30c and i was on the hightway runing about 110km/h then one time i felt the shifting knob is shaking ,the same feeling like there is a spark plug not working and the car going on 3 spark plugs, so i check them and i found oil on 2nd and 3rd spark plug, i cleaned everything even changed the joint-set of the head gasket installed every thing back in place but still doing the same thing .
not all time only when i reach 70km/h on 4th and 5th depend specially when im going on a bridge ,at 80km/h it return no shaking again
1st.2nd.3rd has no problem at all with smooth shifting or burning and the engine gives good power also
so what is that and why exactly at 70km/h ?????
thank you

if there is nothing wrong in the ignition system, there might be a fuel delivery problem.

What eng RPM is this? Can it be duplicated @ that eng speed in other gears? Have you ruled out a chassis or wheel balance problem?

i don't have an rpm gauge in my car but i tried going on bridge on the beginning of the 5th it shaked then i put the fourth still shaking .i know its not a wheel or chassis prob cause when im in the red light i can her the engine "we call it in my country" "spit" but doesn't turn off .
whats wired after two days of shaking it stopped by itself .so i know it the ignition but i couldn't know what exactly