My engine seems to lack power especially at 1st after it warms up for a bit it seems fine e.g. when i push the gas pedal to the floor it has no power at all and back fires but if i give a little bit its fine. This morning when i started driving and came to my 1st stop it shuttered not violently but noticeable i change me fuel filter in November and gave it an oil change im thinking it may be either spark plugs or tranny just hoping to get some peoples thoughts about this thanks in advance