Update. Problem is not with Chrome. The problem is with the way the NEWBB module for XOOPS is storing the viewed topics. Viewed topics are stored at the client in a cookie called "newbb_topics_viewed". After viewing a lot of topics, the cookie reaches maximum size and can't store any more history.

Edit: I ran the cookie test at http://myownplayground.atspace.com/cookietest.html and these are the results for Google Chrome 18.0.1025.142.

12:24:7.64: Starting
12:24:7.316: Max Cookies with Character Length 3 and character "1": 180
12:24:9.488: Max Cookie Character Length using character "1": 4096
12:24:14.238: Max Cookies with Character Length 4096 and character "1": 180
12:24:15.222: Max Cookie Character Length using character "ÿ": 2049
12:24:18.878: Max Cookies with Character Length 2049 and character "ÿ": 180
12:24:18.972: Guessing Max Cookie Count Per Domain: 180
12:24:18.972: Guessing Max Cookie Size Per Cookie: 4096 bytes
12:24:18.972: Guessing Max Cookie Size Per Domain: NA

Results for Mozilla Firefox 11.0
12:27:33.584: Starting
12:27:33.695: Max Cookies with Character Length 3 and character "1": 150
12:27:34.560: Max Cookie Character Length using character "1": 4097
12:27:35.255: Max Cookies with Character Length 4097 and character "1": 150
12:27:36.248: Max Cookie Character Length using character "ÿ": 4097
12:27:36.927: Max Cookies with Character Length 4097 and character "ÿ": 150
12:27:37.39: Guessing Max Cookie Count Per Domain: 150
12:27:37.39: Guessing Max Cookie Size Per Cookie: 4097 characters
12:27:37.39: Guessing Max Cookie Size Per Domain: NA

The forum isn't remembering topics that I've previously read and is showing them as new messages. Has something changed? I remember it doing this before.

Edit:
Must be a Google Chrome issue. I browsed the forum with Firefox and it's working correctly.