Poor internet service provider

I have upgraded to a 4 mbs line in January. Currently I must phone every week to change it back to a 4 mbs line. It keeps on reverting it back to a 2 mbs line. This is 28 July already. I have a 24 month contract with them and can't cancel. The have escalated the fault and said that it is fixed permanently. After a week it is back at 2 mbs