rpm probs

hey guys I was just looking for an answer when I came upon you site I have a 99 Nissan maxima and sometimes the engine shakes a little and the RPMs go down by 200-300 sometimes more I had the same problem before and the shop I took it to said just change the plugs so i did it went away for a bit and now its back i called nissan and they said put some z-max gas and oil treatment in it and if that does not work change your gas filter im stuck please help me
from toronto canada