timer mistakes... is it my computer, connection or just a mistake


Usually a connection and/or site (glitch) problem imo.
Not much you can do other than minimize your side of things that might affect your connection (avoid browsing and other programs running).
sftac

If the problem persists read this:
http://blog.chess.com/NimzoRoy/diagnosing-and-fixing-internet-problems
BTW you should be able to run iTunes and play chess unless you have very little RAM and/or a really old CPU