Frets on Fire: Keyboards vs. Power Chords

Is it just my computer at home, or does a standard computer keyboard not like registering three keystrokes simultaneously? If I see a power chord (two or more notes strung together - in this case just two) I can hold the two fret keys, but the strum key will not register. If I hold just one of the two fret keys, I can strum, but it still counts as a miss.

I'm using a mod called FoFix and the Rock Band 2 theme (goes from an amusing project to something really neat, especially for free) but that shouldn't matter, since this doesn't seem to be about the game, per se, but about the keyboard itself.

Now that Frets on Fire is cool, I fully intend to get a wired Guitar Hero 3 USB (Xbox 360) controller and paint it up real nice (had one, but I painted it Hannah Montana style - pink body, silver neck - for my niece, thinking we'd completely moved onto the wireless ones for Rock Band) and use that. Alternately we could buy the wireless controller adapter from Microsoft for about the same and use the Rock Band guitars, but I never hear about people using those - just the wired Guitar Hero guitars. So I'm not sure that'll work.

Any thoughts on getting the keyboard to work in the meantime? I'm good enough at the game to not be failed out for missing power chords in the songs I've played, but some songs might be impossible with this limitation.