I would also try a process of elimination but I'd get a GMail or Yahoo account and see if you have the same sort of problem.
If you do not see the problem then it's pretty obvious that the ISP is having problems with their server.
If you do see the same sort of problem then the first thing that comes to mind is some sort of packet fragmentation that is caused by your router and/or your connection to the ISP.
Thomas R. Stephenson
San Jose, California
Member of Pegasus Mail Support Team