Packets not being ACKd

I recently had ADSL broadband installed, which didn't work at all. After
some changed to the internal wiring broadband has worked most of the time.

Now I have a problem that some URLs consistently don't work. It will hang
on 'waiting for reply' for a while before it times out. The strange
thing is other URLs on the same host will work. I have this problem on a
lot of websites.

I've discovered that this usually happens if the URL is very long, also if
I'm submitting a large amount of post data or if I'm trying upload a file
I have a similiar problem. If I look in netstat it shows the connection as
having a very high SendQ that never drops to 0 which suggests packets
aren't being ACKd.

I have this problem regardless of web browser or operating system although
I've only tested it using one modem and router. Anyone have any ideas what
could be causing this?