Can anyone make any suggestions about the text size suddenly changing ?
This is usually caused by the scroll-wheel on your mouse - if you have one - finger-trouble
or a defective mouse.
If you're using the touchpad - this being a laptop - there are probably mouse gestures
(leg stroking you finger along one side of the pad) which are being picked up.
Either way I'd go into the Mouse Settings in Control Panel and have a dig round -
disable gestures if you find them . . . . or keep your fingers near the middle of
the pad !