BSNL — broadband not working during night

i have a typical problem regarding my broadband from last 2 months. it works fine between 9am-6pm. after that iam not getting connectivity.i have complained 6-8 times about this but nothing has happened till now. every time when i call up customer care guys to track my complaint status they say that they are not able to track that complaint number. i am fed up with this. can anyone let me know what has to be done