Most likely a routing issue for your ISP to investigate. May be able to remedy it with a VPN. Can try one with their free trial option, and if it improves it is a strong indication your ISP's routing policies need tweaking. You'll need to be in touch with their tier3 support though (sometimes referred to as engineering). It isn't something the tier 1 helpdesk people at the 800 number can address. Nor the guys that come check your lines (unless their actually is a localized signaling issue).

People usually fair better using the online support options over phone support for these things, as it allows you a better way to send them tracert logs and such.