I had similar issues for one whole month. To this day they couldn't isolate this issue. They lco and bsnl enginners tried lot of variations to isolate the problem, finally the bsnl operator went and reconfigured every connections to my lco. Thats how they could fix it but they still dont know what caused the issue of low download and normal upload speeds. The problem is bsnl would never accept the fact that it was a problem on thier side, instead would play the blame game on lco.