Regards my friend!
They will tell you, it's you or your computer or your internet, but the true reason will not be disclosed.
I've seen the occasional "unexpected error" in MP games as a consequence of another player's disconnection, but I understand your description differently. Does it really occur in single play, and more frequently? ?always?
Can you continue the game after relaunching the game client?
A first guess would go towards your connector to the internet, a router or something? As a speculation, it might close down the internet connection after some time of inactivity. You'd not experience much of a difference in connection to other sites, but WGT apparently is extremely sensitive towards the slightest connection interrupts.
Perhaps you might change the settings of the connector?
Other than that, the usual suspects: Flash, browser (each or both), clear cache, re-install... what can I say?
Outside of your control are your provider and the WGT servers.
You might avoid such problems by saving the game or by shutting down the game client => automatic saving.