TCP/IP networking strange problem

This is a weird one and I'm posting this mostly for my own reference so I can recall this when I run across it again. About 6-10 months ago a client of mine was having problems accessing web login pages like yahoo and ebay. I tested from various Windows machines on their network and verified and pulled my hair out for a bit before coming across a tip to check into the MTU setting for the firewall. The internet access is through a dsl modem which connects to a machine acting as a firewall and then to the rest of the network.

So, I set the MTU on both interfaces of the firewall to 1492 (the default was 1500) and everything magically started working. The explanation of this was, our service providers network can handle at a maximum 1492 bits in a packet. (mtu stands for Maximum Transmission Unit) Normally anything bigger is broken up into smaller packets to fit under this threshold. What was peculiar is that this came up out of the blue.

So, last week they have an incident that causes me to swap out the dsl modem with an identical model, same firmware. I duplicate the settings. (MTU on these is controlled by ISP we can't alter the dsl modem/router mtu) Everything seems to work, but since then the same problem has been going on. So, I jog my memory and look. The firewall is still set to 1492. So, I try changing it to 1450, thinking lower would be better. I call and leave a message and look and I've actually set it to 1500 by a mistype. I change it to 1450 and receive a call back. Essentially, "things worked for a minute and then quit again." So, while I was on the phone I had him test again and then switched back to 1500 and one last test. Things were working. My best guess is that the old DSL modem for some reason couldn't handle breaking up the packets. I don't know why the new one couldn't handle smaller packets, that is weird.

If anyone has any suggestions as to why, I'll leave this open for comments.