Di-604 won't let me access Web sites

I got my Di-604 router, rev E over the summer and then finally got it set up with my local cable modem provider, Road Runner. I have two PCs connected to the router and this happens on both computers; Whenever one of us tries to access certain web pages like google, ESPN, yahoo and more, it would show up as "Page could not be found" and we would have to refresh it or click in the address field and enter to force it to retrieve data from the sites. I checked around and some sources said that I have to lower my MTU to incrememnts of 10s to find out which one is the correct transmission we should use. I lowered it to 1492, 1482 and on till I found that 1472 works then I had to restart my computer, then 1472 doesn't work. I lowered it again to 1442 and it works, then restarted and it won't work. 1352 works then restart and it won't work. Obviously it's not the MTU issue.

Anybody having the same problem as I am or could it be a faulty router I'm using right now? If I am not clear on this problem, please let me know. Any help is appreciated! Thanks!

No, when I am directly connected to the Internet, it works fine, no "Page could not be found" errors and such. This happens only if I am connected through my router to the Internet. My fiancee's computer is running Windows 2000 and she get twice as many errors than I do so this is an odd behavior and I've tried to do many different possible solutions and nothing seem to work.

BTW, sorry for the late reply, I was out of town for a few days and now I'm back. Thanks again in advance for your help!

I thought about updating the firmware again after doing that back over the summer but it didn't work. Yes I am using DHCP to assign IP addresses and the IP address of the router is 192.168.0.1. I also don't see where I can configure the DNS to my ISP's DNS server. I am using Time Warner's Road Runner, btw, and I do not know their DNS server's address. The repairman came here and he said he do not know how to configure the router and he only know how to set the computer up directly to the modem.