My '05 STi did the same thing at low speeds. Like everyone else, only when I was holding a constant speed. When it first happened to me, I was driving uphill and I initially thought I had driven over a small patch of ice and lost traction momentarily. But it happened again on a flat section of pavement and it legitimately felt like it lost power for a split second, like fuel cutoff or something.
Only happened that one time though. My car is stock aside from COBB's stage 1 map.