Internet Traffic To Our Site Hampered For Certain Countries Only

This is not strictly a technical issue with our server as such, but this subforum still seemed most relevant. If not, please move the thread to a better suited one.

Bizarre problem: in the last 2-3 days, visitors from certain countries (France, Holland, Serbia, Slovenia, Macedonia, Montenegro) are having difficulties in reaching our website hosted on cari.net. However, other countries (USA, UK, Germany, Russia) don't seem to have this problem at all. The server itself is a-OK.

The website hosts an IPB forum. The problem is that every so often (very often), upon clicking on a link, the browser hangs on "connecting", and often times out and returns a Bad Request message. Sometimes, when a clicked link that's waiting is refreshed, it immediately loads the page up quickly, as if nothing is wrong.

We cannot call our hosting company, as it's really nothing to do with them, the server works perfectly, it's just that traffic from certain countries has difficulties to get through a certain level3.net node in the USA (that is where tracert usually breaks down, or gets through after several attemtps).