Based on my experience, if you make enough complaints (they have a record of your complaints), then you can cancel the service if you agree to pay penalty, which is usually 400-600 HKD. That's what I will do with my phone. However, in my case, I was not receiving the service, so it's pretty straight forward.
In your case, the problem is that you do receive the service, and you would have to prove that it is not the speed you're paying for, which is harder. Whenever your access is interrupted, make sure you make a phone call, and after 3 times you can demand to cancel the service. It should work.
Originally Posted by syedagha