With litecoinpool.org is very unstable. With default settings it starts at 3.5MH and after a couple of hours I get 0.1-0.3 MH.

Can someone help me to configure it at maximum? With stable performance?

An other things I got this warning:
I saw a warning: failed to sanity check in lowl-vcom.c windows_usb_get_string():508


Edit:
OS: Windows 10 64Bit