It's probably the timing. Although, it could be shacking because of a lean mixture [too much air]

check the air intake to make sure theres no holes in the pipes, as this could be letting more air in than the engine it meant to be reciving.
LIP Reviewed by LIP on . car experts, help me out...my car is shaking i drive a 2000 nissan maxima SE automatic. lately when between 600 (idle) and 1200 rpm my car starts shaking/chugging/hesitating...whether in gear or in neutral. when it gets above about 2200 rpm the chugging goes away. the chugging is at its worst when i'm in 4th gear travelling around 37-40 mph and at 1300 rpm or so. when i turn on the a/c it gets worse...the chugging gets stronger and slower. when i turn off the a/c it gets weaker but chugs more rapidly. my car has lost a lot of power Rating: 5