That shouldn't be the problem as it's a brand new computer with still very little installed on it. It was upgraded to 1024mB of RAM, and appeared to be jumping from day 1. At one point I discovered a setting somewhere which was to optimise display for LCD monitor which appeared to work. After some problems with a wireless network, I had to do a system restore, and I don't know where I had found that setting anymore! D'oh!!!!
I shall try that suggestion from cypherus at the weekend... thanks.
Meantime, any other suggestions most welcome.
Thanks everyone.