Fibre speed not as promised

To be honest... Their sales and after sales service is unmatched they have a fantastic turn around time and handle issues almost immediately... However ... The reason for choosing them as a service provider for fibre the the home was for the constant and in interrupted fibre speeds. My dilemma... For the last couple of weeks I noticed when performing a speed test from around 6pm to 10pm the speeds are terrible it's almost 20 % of what I signed up for and in most cases the movies can't even load... I will allow them to fix this but they cant expect me to happy with this forever please fix it or I'm forced to move to another service provider Thank you