LEC805

my rx8 is a 2004 6-speed manual transmission. i noticed that the whole car shakes whenever the rpm's go around 1500, just above the 1 on the rpm guage. it does not shake at all if its above or below that part of the guage. its weird because for example, when i accelerate and the rpms go up, it shakes only at that point even if i accelerate fast. and the same when they go down, once i put it into neutral and the rpms go down, it does a small shake wen it passes just above the 1 on the rpm guage. its been getting a little annoying and i was hoping someone could have a solution. thanks (: