Driversprep.com has had intermittent network issues since December 29, 2012. For short periods of time the site has been unavailable.
According to the hosting company this is caused by distributed denial-of-service (DDoS) attacks.
DDoS attacks are an attempt to make a server or network resources unavailable by flooding it with traffic.
Even if the attacks have been targeted against other sites, they have affected the network and all sites connected. Collateral damage, if you will.
When one of these attacks occurs, response from the site is slow or there is no connection at all.
After 15-30 minutes the hosting company has usually managed to block the attackers. At least until the next attack.