how modified is the car? My car does it sometimes too, i have minimized it by tuning, but it still exists. I read an article explaining that the design of the rotary allows exhaust gases to mix with the fresh mixture in certain situations and it causes a misfire and results in the bucking. The technical name is 3:3 ignition mis-fire. here's a link to the pdf.
http://foxed.ca/rx7manual/manuals/RE...amoto-1981.pdf