Slow FROGFOOT fibre downloads after 7pm

Installed a 500MBPs Frogfoot Fibre 2 weeks ago - brilliant at first for last few days every night after 7pm the download speed drops to 25MBps or maybe 50MBps - not sufficient to watch Netflix - Afrihost as the ISP log calls but we never get an explanation - I am very disappointed - Frogfoot does not answer phones - Afrihost just shrug their shoulders - not sure if I should revert to the slower Openserve :-(