Often unable to load web pages.

Question

My MacBook har suddenly decided to not load web pages ...
I surf around, and maybe one fifth of all the web pages won't load in my browser (both Firefox and Safari). It isn't something about the web pages, because after some time I can get them to load perfectly. I contacted my ISP, and they couldn't find anything wrong. Besides, I can surf just fine from other computers (non-Macs).
I wondered if it was something about the port, since all other apps work perfectly (for example Google Notifier and Audium is always connected), but https doesn't any better. I've also pinged web sites for a long time, without any evidence of something being wrong. One strange thing is that traceroute sometimes gives me "no such host," and then a moment later, it works with the same address.
It's not Airport either, because I now use a network cable and have the same problems.

Answer 1

There are several possibilities that come to mind.
Since HTTP transfers often fail, but lightweight protocols such as AIM don't, my first thought is MTU (Maximum Transmission Unit).
The standard MTU for ethernet networks is 1500 bytes. This means that all transfers are broken into chunks of 1500 bytes each before being sent over the network. In some cases, certain network configurations (such as DSL links that use PPPoE) require a lower MTU such as 1492 or even 1460.
Normally this isn't a problem since the router tells the client what MTU to use but sometimes this doesn't work - especially if you're using a NAT device.
When there's a mismatch your Mac doesn't know that it needs to use a lower MTU so sends traffic upstream at 1500 bytes and the router chokes when it tries to cram that data into its 1492 link.
This is why lightweight protocols often work - the typical AIM packet is less than the MTU and therefore doesn't need to get truncated. However, large file downloads exceed the MTU and run into problems.
If this is the case you can force your MTU to be lower using the System Preferences -> Network -> <interface> -> Ethernet tab. Switch from Automatic to Manual and you'll see options for changing the MTU. Try lowering it to 1460 to see if it works. If it does you can try increasing it until you find the optimal value.
If it wasn't for your statement traceroute sometimes gives me "no such host" I'd be sure this was the problem. However, MTU doesn't usually cause DNS failures since most DNS requests are small.
If your DNS is failing and the ISP belives the DNS server is working correctly, that hints at a link-level problem. Packets are getting lost somewhere along the link between you and the server.
The first thing to check here would be speed and duplex. The fact that you've used both AirPort and wired connections hints that it's not a problem directly on your machine, but what about the rest of the links between you and the router? I'm guessing you have an ethernet cable between your base station and a switch, maybe another cable between that switch and the router? Either of these cables could have a mismatch.
Without knowing the switch it's impossible for me to tell how you check the switch port configuration - some switches have LEDs on each link that tell you the speed and duplex, others have a command line or web interface that will give you port status and statistics. If you have such a switch, check it. Also check the switch logs if you can - you'll often see error messages such as 'excessive CRC errors' which can indicate a duplex mismatch.
If you can't get the information from the switch, try manually configuring your devices. The same preferences tab where you set MTU can also be used to set speed and duplex. Just be aware that if you choose a combination that the switch doesn't support (e.g. you choose gigabit but you're connected to a 100mbps switch), then the link will go down.
Try different options to see if any of them work any better for you.