Bad service experienced with 300GB capped package.

I'm a customer of Vox Telecom with account ID ***. I signed up for their R485 300GB capped package on a 4mbps line which is supposed to be unshaped, but I've been experiencing very bad internet speeds. During the day it seems to be running well but after 18h00 the speed is horribly slow, and this is the time when i normally use my internet the most. I called Vox to report this problem before, they insisted that the issue lies with Telkom. I've called out Telkom a couple of times, they have tested the line thoroughly and cannot find anything wrong. If Vox cannot have this sorted I'm gonna have to start looking for another ISP.