This sounds like a DNS problem. This is an issue we have seen before and it may mean they are using a certain manufacturer of infrastructure router that has a bug with its DNS server.
It caused us a lot of headaches until we discovered it and worked around it. it tends to affect yahoo, amazon , some google sites etc..
A workaround there would be to set your DNS servers on your MAC to say 188.8.131.52 and 184.108.40.206 instead of the ISP provided ones.
If that does not resolve it then they may be using a thing called PPPoE and the issue may relate to MTU settings as this can also cause this effect.
Either way you need to poke Kencomp , but at least if you discover it is DNS then you are armed with that fact.
Bill Lewis - MD
Kijoma Broadband - (Division of Kijoma Solutions Ltd)
Fixed wireless ISP - ISPA/CISAS/RIPE members