Speed drops, but the complaints are ignored!

Every few months the ADSL line at my parents' house runs into problems with speed. For the past month, every day from 6pm the speed drops to 0.5 MB per second with ping response of 300-400 MS and jitter of about 100ms. The usage of the Internet becomes unbearable and my parents cannot use it. Everytime they phone they're told there is nothing wrong with their Internet despite evidence to the contrary. Why is MWeb not fixing this? My parents are retired and cannot afford fibre. I suppose it's easy for the likes of MWeb to dismiss pensioners! Disgusting!