Quality of DSL

My contract for DSL is My HOME L 100Mbit.
But its speed was lower than 4Mbit during the weekend(from Fraiday to Monday).
May be there are some traffic problem.but even though traffic problem. quality is not meet the contract.(should above 50Mbit in worst case)

If the O2 DSL has the same level of quality over and over again, I have no reason to keep the contract at 100Mbit.

I want to know what action I should take if O2 can not keep the minimum quality stated in the contract.

Regards,