Right cursor key causes weird behaviour

Pressing the right cursor key sometimes causes the on-sceen cursor to jump by two spaces rather than one. Tried loads of ways to replicate it in order to pinpoint the cause, but seems totally random. It happens about one in every ten presses. Really irritating. Tried different OS's but same for all. Anyone else ever had this problem? Any ideas for solution?

Mr O

sometimes my keys stick, turn off unit and press quickly for like 20 times. turn on try again.