Problems with buffer setting

Hi, I have a problem regarding the buffer settings of my sound card UA 25 EX. I can not set it to a value greater than 960. At this value the more complex songs on Ableton Live sound disturbed despite the stress test gives a positive result. I own a Fujitsu Lifebook A512 Notebook (Core i3 Processor, 2.40 GHz, i3-3110M, 64-Bit, 4 GB DDR3 RAM), so I do not think it's a problem of insufficient resources. How can I set the value of the buffer at a higher level in order to try to solve the problem of noise?
Thanks in advance.