Here's the problem:
Sometimes web pages will load up fine, but sometimes only some will be loaded or none at all. The connection seems to be reset before transfer is complete. I'll often have to keep resreshing the page for it to load it all up.
Downloads will suddenly stop part way through, and have to be restarted. It seems that data just stops transferring, but the applications think there's still a connection.
Sometimes it gets so bad, I restart the modem-router and it gets much better, but it happens after a short while
Connecting between PCs on my LAN is ok (wireless-to-ethernet etc) So it seems to be the modem part of the modem-router.