start simply... make sure you don't have garbage software on your pc (spyware, etc...)
next, check the device in device manager... make sure you have the latest drivers.. also, make sure the bios on the pc is updated as well...

lastly, check the pc resources... make sure you're not running things that aren't necessary... what could be happening is that the more you record, more is going into memory before it can be written, if you have too much stuff loaded, it will lag....