Slow Evening Speeds

[BerryHouse]

I purchased the $89.99 Unlimited 'Super Fast' Plan on NBN100. My evening speeds, however, are nowhere near the advertised 80 + mbs. Instead, it drops to below 4 mbs from 8PM until 12AM, both plugged in and wireless. I have restarted the router and the NBN box twice to no avail. 

I want to know why this is, and whether I need to switch providers.

[BerryHouse]

Ongoing for me. They did something externally and the speed improved, however, it has since deteriorated. I have tried contacting them since with no success.  Either NBN is a scam service or TPG is not properly assessing houses for data plan eligibility. If they cannot provide 80mbs evening speeds, then they should not charge us the premium.