I kinda have "what happened" figured out. Not the problem, just "what happened".
You see we had a major "lockup" twice. And the resolution at those times was to turn off vBseo.
Well, vBseo modifies forum URLs. It gives search engines "friendly" names to better find stuff. It changes forum numbers to nice names in the actual URL.
Anyway, when I originally turned it off, the site had been down, so when everyone came back, your browsers refreshed their cache automatically. So users did not experience these broken URL issues at that time.
Then when I turned vBseo back on yesterday, everyone's cache still had the previous visited URLs saved in it's cache. That's why forums you regularly visited did not work, but forums you had not visited lately (were not in your cache) worked. If you took a "different than usual" path to the broke forum you had previously visited, you could get to it cause it was not in your browsers memory.
I've actually debated leaving vBseo off, but the "LIKE" system is in it, and any vBseo created URL references to images where someone copied it to another post, or signature, would not work. Hence some broken images when it's off. So, we kinda are stuck with it. That means, I have to identify the specific problem that the raffle system has with vBseo (the two lockups that we had). If we get another lockup, I'll be turning off the Raffle system instead of vBseo, and we'll see if that's provides a temp solution. In the mean time, I'll do more Internet Searches for a permanent solution.