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The research, from network management firm Parallel, found that the ISP sites surveyed were down for an average of eight hours and 25 minutes in the quarter, which equates to more than 33 hours a year.
"The overall availability of ISP sites is fairly good compared with data gathered about top companies in other industries," said Parallel director Tim Moore. "However, the availability of some still leaves much to be desired."
Parallel also found that 80% of the ISP's Web sites had a home page size of more than 40 Kbytes, which meant download time was always more than eight seconds for dial-up modem users.
Web designers recommend home pages should be no larger than 40Kbytes to give home users - 89% of whom still use a modem connection, according to telecoms regulator Oftel - good performance.
The ISPs performed badly in this area, according to Parallel. Researchers found the largest ISP home page size was 151.6Kbytes, while the average was 76Kbytes.
For a home user, this means an average wait of 40 seconds for the home page of their ISP to download.
"There is no reason for the home page to be so large," Moore said. "Most of the services are not going to be there so users should not have to wait this long for it to download."