Yeah, I have a cheap wired keyboard also and I get the same problem. It worked perfect when I played on PS3 and now a few of my letters don't appear if I'm typing fast. I basically have to hunt and peck my typing and re-read what I type every time because of missed letters.

What's weird is it seems like it only happens with certain inputs. /fc is ALWAYS just /f if I'm typing too fast. /p usually ends up just being a /. Space bar cuts out sometimes. And I literally have no other problems with any other letters on my keyboard. Plugged it into my PC and wrote about 300 words on a Word document and not so much as a mispelled word, so I know nothing is wrong with my keyboard.