I'm afraid I'm out of ideas for things to try. If anyone has a suggestion, please let me know.
For what it's worth, you may want to try to see if there was something different between this post and about 2 hours ago.
When I first connected, it was great, and it's gotten worse, although still manageable (and better than what we've seen: I just had 2 errors 503s).
One of the things I would do is analyze logfiles to see what was going on around the time of posting some of these reports. All of my posts, except this one were contemporaneous to some period of difficulty. Log files should be able to tell you what the machine is doing at that particular time, and/or who is trying to access it at that particular time.
I'd ask the service provider to take a very close look at any and all updates or modifications that were done in April and early May.
They may have told you they haven't changed anything, but they may not have looked closely at it, to save themselves the work of deeply diagnosing a problem.
Just had about half a dozen 503 errors. It is 12:30 Eastern Time as of this posting.
Last 15 minutes performance of the forum has been pretty slow, 3 or 4 503 errors.
Michelle, is your webmaster back and are you able to look at log files?
I will say that nearly every morning at 7am Eastern, performance is pretty bad. Also seems to get bad for a period of time around the noon hour eastern time. And, again, this began on May 13th. Is there anything about your ISP can do about dropping packets requests from known bad sources before they ever get to the webserver to be processed by the .htaccess list on the server?
Forum is sluggish and extremely slow. 20-21h GMT+1 time. A 503 error as well a while ago
Xavi
EDIT: just got another 503 error when trying to post here
Yesterday I noticed something weird about our main website. When I hit "view image" on any of our product images, instead of giving me the full-sized image it gave me a thumbnail with a weird address like this: atlas-games.com/images/produ ... WtrdMj.jpg
I went looking for an explanation. Turns out it's an Apache module called mod_pagespeed that's implemented by our host ... this is the "Page Speed Optimization" we turned on to try to fix the forums. The interesting thing is that in the comments on our host's announcement page for this feature are a lot of reports of 500 errors, failed page loading, and memory usage spikes, plus XHTML validation errors. Sound familiar?
So I'm turning it off on all of our domains, to see if that helps the forums. Let me know if it suddenly improves. Thanks!
Right now, performance isn't bad.
Between the time you posted the above post, Michelle, and now, the forum did have a period of extreme slowness followed by a period where I received 3 or 4 503's while attempting to make a post in the Bibracte forum. So, I'm not certain that the module is responsible for the forum woes.
For the most part, it's been going well (and I've been on since about 12.30 pm). Although I did have a spate (around 1.30-1.45 pm Central time US) when I had about four or five "Service Temporarily Unavailable" messages in a row.
I have a very difficult time connecting to the forums. It takes a long, long time (nearly a minute) for a page to load when I use Mozilla Firefox or Internet Explorer. Using the Google Chrome browser, I can't connect to the forums at all, and when I click the link on the main page I am simply returned to the main page. It seems like it has been this way for weeks with no change.
Matt Ryan
Performance yesterday was awful until later in the evening/night 6pm and after. I was able to get a few posts in during the day, but lots of trouble and effort to do it.
Performance today is not that much better than yesterday around the same time. A bit better, but not much.
Matt: I don't think it's a browser issue. The forum issues have been ongoing since May 13th.
How have things been lately? I'm thinking about taking down the Google ads to see if that helps, but we don't want to lose the income if that's not really the trouble.
Haven't had an error message for the last week or so. Performance has been reasonable as well, that I can remember.
Same here. The forum seems to be back to normal. 
It is better. One thing I'm curious about is if any usage statistics for the site have changed. If the forum was subject to an extended DDoS attack, and they just moved on? If you care to share that information.
It's been thankfully stable for me for, I think, about a week now, too.
Well hallelujah! Obviously not because of anything WE did, though. shrug
Wasn't the google BOT thing something that made it better?
It might also be a case of limited bandwidth per month. July 1st reset the counter for that, if this is the case.
The problems began in May.