My laptop is set to 'do nothing' after closing the lid, and yet Tixati transfers practically come to a halt whenever I do that. Any ideas why? Thanks.

Closing the lid can affect your WiFi signal causing it to drop out (assuming you're using wireless).

Try changing the advanced power options in the control panel. Most likely it's set to conserve battery power when the lids closed .I set to use 100 percent power even on battery when not plugged in .