Same thing happens with me. And after attempting couple of CAT tests on my laptop, I realised its actually the ticking clock on the screen which is making me nervous.
Though it may sound weird, but when I attempted these CAT tests, with the on-screen clock hidden and tracking the time with my
wrist analogue watch, I was performing much better. Maybe, since we are very much used to of those traditional exams, in which we generally use wrist watch to keen an eye on timings. With wrist watch, it was easier for me to make out the time left, spend etc, by just giving a quick glance on the dial.
Anyway, can you let me know if wrist watches are allowed while giving the actual GMAT ? Probably I would try the same strategy out there.