Vista startup: intermittent stutter

I have a Sony Vaio, which came with Windows Vista Business installed. When
booting up, the Microsoft Corporation bar, with the green flashes running
along it, is sometimes missing either its lefthand or righthand border. The
login screen might then come in three quarters complete and a quarter blank
for a second or two, then complete itself. This an intenittent issue, and
does not affect the normal functioning of the computer, which is otherwise
flawless. My concern is that it may indicate a problem which will only get
worse, and which will eventually become a real problem once the warranty
expires!

I wondered if it might be related to the fact that I installed XP, using a
procedure for my machine detailed on the Sony website, and which involved
downgrading the BIOS. After a short while, I then decided to reinstall Vista
from the Recovery partition, and upgraded the BIOS again. I thought maybe
something had not reinstalled properly, but I'm not technically competent
enough to know if this is likely. Sony support suggests I reinstall Vista
again from the Recovery partition, but I don't want to do that unless I have
to, and unless I'm convinced that it's likely to help.

I'd be grateful for any advice from users on this forum.

Many thanks.
--
Keith Wilson