05 accord engine vibrating?

05 accord v6 at 82k..engine was vibrating like crazy as if it will jump out..it was misfiring very shaky in the car..engine wudnt start in first try..i had to hit the gas and turn the key to get it started..wud d turn off wen i was at a redlight..took it to the mecanic his first guess was the spark plugs or coils..he sat in the car hit the gas hard in neutral and told me it wasnt plugs..he asked me to put 3 quarts of dry gas and drive..at first it was d same.it was gettin very bad..engine was shutting down more at every stop..but after driving like 80 miles i noticed it wasnt vibrating anymore..it got the power and cud barely tell the difference for hesitation.So does it mean my gas tht i filled up last time was bad?it was bp 93..thts wat i use all d time..ps filled up in the morning had problems starting in d evening..i thought to change the spark plugs regardlessafter this thing is solved