You have used the dslchecker link I posted and that shows estimates for your connection
you need to run btspeedtester diagnostic test with a wired connection and post the results when you feel your download speed is low
so line appears ok have you tried btspeedtester tonight to check download speed?
Run again tonight, and got this result:
Guess it's stable again after the blip last Friday and Sunday!
Download looks ok
If it happens again just post back with btspeedtester results