Downloads pause and time out frequently
Solution 1:
Typically you can work to isolate and solve the problem, by systematically proving good various parts of the network. This is a process of being confident to say, I know this works by using the appropriate tools to investigate, and by parts you will arrive at some final piece of the jigsaw and say, I know this is the problem, because everything else is good!
If you can replicate the problem in devices attached to both ethernet AND wireless then that isolates the problem in the final link between the network <=> Cisco 1811W <=> DSL Fibre <=> ISP <=> and the Internet
If you only see the problem in either wired network OR wireless devices, then you can target the wired ethernet or wireless configuration on the Cisco 1811W. Then you can can review the settings common to the problematic segment as a next step.
Generally Reseat any commonly linked ethernet cable, and try swapping the DSL cables if available, when testing some device.
Check the MTU and auto-negotiation settings on the router, that are set for the DSL, review the router log file from IOS.
The router will be running IOS 12 or something like that, which will have some good command line tools accessed via ssh for checking negotiated settings.
Use the show interfaces command to review error statistics such as resends and dropped packets. It might even have a web interface (but I am not working with cisco IOS devices at the moment so this is not tested just from some notes I made on trouble shooting cisco networking)
However you should be able to pull up a table of per port error statistics from the cisco console using
# show interfaces status
# show interfaces counter errors
and for a particular port e.g.
# show interface GigabitEthernet 5/28 status
# show interface GigabitEthernet 0/24 switchport
Edit: here is a little video of some guy showing how to use the ios "show interfaces counters errors" to troubleshoot problems. It is actually really cool, but its probably in too much depth, but it gives you the information required to detect duplex mismatch, or auto-negotiate settings.
p.s. you can prove the router part of the connection, by plugging an alternative DSL router into the fibre connection, if downloads work find them, you know the problem is this side, rather than ISP side.
Solution 2:
Some ISPs make the strange decision to block all ICMP packets on their switches or firewalls. This blocks calculation of the Path MTU, which means you get more fragmented packets occurring as they pass through routes with lower MTUs. Maybe you are seeing the result of this.
Fragmented packets have to be reassembled which can be a problem if you also have packet loss! Given you are trying to download large files, both fragmentation and loss of packets will be a greater problem. Path MTU discovery is designed to reduce fragmentation.
So how do you know if your ISP has done this to you? You could ask them - however, in my experience ISPs will far prefer to send you off with basic troubleshooting for several days/weeks rather than admit they might have done something wrong. And of course sometimes they are right to!
You should gather information to show them what you are seeing. Packet Captures like you have done in Wireshark or collected at your firewall are helpful as they often reveal the level of fragmentation. You can check whether path MTU discovery is working using tracepath (*nix) or mturoute (Windows).
If you do find pMTU is not working, it could be either your ISP, or the ISP of the site you are trying to download from. If you see the problem for downloads from multiple sites, chances are it is your ISP.
And of course, it could be a whole bunch of other things too :-) Good luck!