Mag 250 freezing?

Hi, my connection is 15mbps/down 1,2mbps/up and the stb freezes quite often. Using the mag's speedtest option results from 500kbps to 4mbps max during evenings (mostly 1-1,5mbps) and 8-9mbps during mornings. As you understand my problem is in the evenings. Why is such difference from 15mbps in PC to 1mbps in MAG at the same time? My sister has 5mbps in PC and her MAG never freezes, on another provider though. Please enlight me..