Could be a slow site, or a busy time for your ISP, or interference on the phone line
When you say it's normally 26-28kbps, is that your quoted speed from your ISP? If so, that's in kiloBITs per second, and your download speed is shown in kiloBYTEs per second
As a bit is about 1/8 of a byte, your speed will be around eight times slower, plus internet connections are never as fast as they say they are so that would be about right
For example, I have 1mbps broadband and can download at a max. of 130kbps
Hope this helps