This is certainly an odd glitch. I do not currently have this problem, but I can tell you what causes problems with my PC when it comes to this lag.
To start, upgrading (more like downgrading) my PC to Flash 10.1 KILLED FFR. I could barely play a single song without it all of a sudden jumping frames like crazy and giving me goods absolutely everywhere. I currently use the
Adobe (Macromedia) Flash Player 9.0r260 standalone to play FFR, and generally experience 0 lag.
In addition, enlarging the FFR window to a much higher proportion (zooming in, and full screen) causes me to get random goods in quite a lot of songs. Try playing it on it's default proportions in a maximized window and see if this makes a difference.
It could also be your monitor. I noticed a significant decrease in the ability to play FFR on my upstairs computer when I purchased my 22.5" AOC, and upped the resolution to 1600 x 1200. The computer I use FFR on has only a 1024 x 768 resolution, I'd assume less processing, less lag or random goods.
I am unsure of how many people are actually experiencing this issue with random goods/misses/boos/etc, but if it isn't a PC issue it should definitely be looked into.