For example, if the file is downloading at 4Mb/s, there is trouble if audio is playing and seems to also have trouble with other resources like the mouse. I have an ASUS G55VW with 12Gb of RAM running Windows 7 64-bit. Do you think Tixati causing this? I searched for similar postings, but found no solution.

Might be generic or bugged drivers; Windows drivers do cause this.
Tixati do not have such an bug, its an external to this software bug.