Fibre internet feed is ultra erratic.

On weekends my fibre internet feed is commonly very erratic. I log a web complaint and on Monday morning I receive a SMS stating the problem is being attended to. The next SMS states that they have checked my line and found it to be working well. Yes it is working well on Monday but on the weekends I sometimes get download speeds of 0.04 to 1.03, my line is 20 meg. This has a negative impact on what I do. Why does it fluctuate like this? Please advise as the problem must be solved.