Internet is slowing down need help?

Sir,
I have an internet connection whose speed is 256kbps and the special feature for my net connection is speed is normally more than the normal (256kbps) before of 11 th august means 15 days before it gives approximately 1.888mbps (it's the maximum speed)but now it's down to 686kbps normal how to make it again speed up to 1.888mbps please help me

Thanks,

Regards,
sudheer413