[Mamady]

I wondered about this for years - because I do it all the time.

The answer finally came to me from trial and error of something else. This is caused by caffeine - usually from coffee. Engineers dring lots of coffee, so often suffer from this. It is a type of "jittering".

[uriloran]

Maybe that's the answer for you. I nearly never drink any coffeine and still do it since my childhood.

[skopp]

Gawd I love coffeine