I have a WorldCall connection which is of 2mbps continuously throughout the whole day and night. Recently during the day the speed has become very slow and I get 256kbps speed. The 256kbps speed which I get lasts from morning to late evening and its only at night when I get 2mbps of speed.
I had a WorldCall broadband connection some years ago, I too faced a similar speed problem. Once the problem used to get resolved it started again after a day or two, so I got switched ISP's and got PTCL broadband instead.
If repeated complaints to your ISP has yeilded no results then it is time to switch to another company in my opinion. The problem you are facing is most likely caused by network congestion and there is not much you can do from your end.
The reason for the slow speed during a particular time maybe because of the high number of users during that period. Although there is not much you can do, but you can still try by calling WorldCall customer care.