Vodacom fibre is really substandard and unreliable.

Every month and during the month my fibre speed goes down. I pay monthly for a 20 Meg service uncapped. Towards the end of the month around the 20th of each and every month my speed is drastically reduced. When I call the customer service line they are always very helpful and immediately my speed is restored only to have the same problem the following month. Today however was different. I once again phoned the help desk and my speed was restored only to be back down to more than half an hour later. Can anyone help?