300gb fat pipe unshaped speed drop

I am a Vox customer who got a 300gb unshaped package on a 4meg telkom line. During the day my internet speeds are fine however after 6 in the evening it drastically decreases. I am a gamer and between 6 till 12 in the evening I need a fast internet connection for my gaming but now I'm experiencing lag. And when I do a speed test its well below what i'm supposed to get. I've contacted vox which refered me to Telkom, however telkom have done numerous tests on my line and cannot find anything wrong. This leads me to believe that it must be an ISP problem. My account nr is ***. If this problem persists I would have no choice as to try another ISP.