Please upgrade to CB 1.8 and confirm if issue persists.

Do you happen to be using some sort of 3rd party cache or have very long Joomla cache times? What's likely happening is the spoof string is being cached and on next registration the old spoof string is used so it's not validating. Please disable then purge all cache and confirm if issue persists. I don't recommend cache times any greater then 5 minutes, but 3 minutes is my recommendation.

You can use caching, but don't cache for real long. A lot of CB, including its security, is very dynamic and dynamic content in nature is not cache friendly. Caching is helpful for sites using basic features like Articles, but if you've a community with constantly changing content you can't use caching or have very long cache times as it'll constantly result in out of date data. Set the cache time in Joomla global configuration to 3 minutes, purge all cache, enable the caching plugin and it should be fine.

It's also worth noting the caching is not going to improve your performance much if at all. A slow site is usually due to bloat (installed far too many extensions, seen sites with as many as 30+ extensions; bad idea) or bad hosting ($10/3month sounds great, but is often garbage and ridiculously slow; never buy into "unlimited" claims).

You should profile your site using Firebug for example and find exactly what's slowing it down then remove it or fix it to be faster. Profiling your site will tell you exactly what files are loading in and how long they take to load. For example on quick review of your site many of your images are PNG format. PNG is very slow to render. You should provide them in JPEG especially if you don't need transparency. Your logos and such alone take approx. 20+ seconds for them to all load in and that's a lot of time.