Holding down movement key doesn't repeat

I have a player with a weird problem, and I am stumped.

All his movement keys--WASD--work fine. Except that W, for foward, only works as if it is pressed one time. Holding it down causes the player to move forward one step, and stop. AS and D are fine.

Remapping moveforward to another key has the same result.

What on earth could this be? None of the other players (that I know of) have run into this problem, including me.

Getting into a car or an airplane has the same problem.

Any ideas would be greatly appreciated!