a "minor" shaking problem

okay, this problem seems to come and go off and on

i've only had the car for like 4 months, tuned it up, babied it

anyways (99 Grand Prix GT)

Sometimes when i start it, it'll get a good shake going (could probly enter it in a cheap dance contest

i dont know what the problem could be, but after i rev the engine for a while, its stops. (like 3 secs. if that)

its not a real big problem with me, but i'm wondering what could cause that

and theres been no SES light