you should try using mtr.
http://www.bitwizard.nl/mtr/
when you start crossing providers you get into peering type problems where it's hard to tell who's at fault unless you can test from both networks. also time of day and physical location are factors. and don't forget that it could just be that their .com site is slow and not the actual service. try hunting down an ip of one of their users.
given that it's presumably your only alternative to dial up i would just go with it and make sure you get some kind of sla.