I have had intermittent glitches with my keyboard but not the one you describe. I asked a young and well informed sales rep at my local computer store for advice. This what I learned: not all keyboards are alike. Differences in contact-switches and the matrix layout can make some cheap keyboards unreliable. Better hardware may help. Yes, this sounds like a sale-pitch but I know enough about electronics to know that quality of parts and design can make a difference. I have had great keyboards that allow many simultaneous key inputs at once (PS/2) but I have also have crappy ones that fail to register simultaneous keystrokes reliably.
He also spoke about potential buffer problems with cheap USB keyboards. Sharing the USB may lead to slight delays processing your key-strokes. He said that older PS/2 connections were not prone to this issue and suggested using an adapter for my USB keyboard.
