No, we are not alone.
I'd like to share with you and everybody some of the results of my attempts to solve this. I hadn't mentioned the provider-host for my site. It also is GoDaddy. I made a lot of calls to them, talking with a different tech each time where none of them were willing to admit the problem is on the GoDaddy end. I came back to WordPress and as you see in this thread one of the WP knowledgeable folks posted the skinny about WP. She's on point with her analysis. Here's what's happening with my instance: GoDaddy has timeouts set on their servers. This is a default. I found the actual screen shots for this setting on a linux server, on the web. This timeout is set to 30 seconds by default and GoDaddy utterly refuses to change it , not realizing or caring that it will improve, not degrade, performance(ultimately it helps sell more expensive service packages, upgrades. Why not just advertise limits instead of effectively throttling?). Most of the tier I support people are totally unaware of this, themselves never being server administrators. What this setting causes is when you have a lot of pages, and especially if there are a lot of queries, speed slows to the timeout limit and generates a server reset.
OK. I have a site that started with over 6000 pages and is planned to become rapidly twice that size in pages, then add posts on any, or all, pages, with custom database calls that were, and are, not a part of any theme - entirely developed by, and written by me. I ran into the same problem, and thanks to GoDaddy techs either not knowing, and/or their refusal to up the timeouts, I was left with solving it myself. That solution led to their losing this customer for that service.
First, solution - I moved the site to another host. Didn't have to move the registration, just the hosting involving only a DNS setting change, and of course buying, and setting up, the service. I now host this site at justhost.com while GoDaddy maintains the registration. Called around and got the responses from several hosts and they were one of the least expensive that explicitly said they have no page limit and have their servers and services set up not to timeout for normal processes. They even offer some options on optimization. Both services an troubleshoot down to the line event.
So what was the actual problem? In my case the timeouts occurred at nearly 5000 pages. I deduced this by starting with a fresh, clean, WordPress installation, with no plug-ins or widgets, using stock 2011 and 2012 themes, then added a couple of extra random themes for testing. I started importing pages incrementally until I got to 5000 pages and where the resets began. Anything less it would work, but slow after about 3000 pages. My limit of 5000 pages may have been from having them change my php.ini file so it used more memory, but that's on their end and they didn't want to do it - I can be a persistent b******* when I wanna be :). This was done on the local side also with a slight increase in performance. I'm running xampp.
In your case you might be passing some serious queries making it timeout with less pages but same load. These timeouts are dependent on load and not number op pages. Depends on how your theme(s) and your custom code are operating. Plug-ins, widgets, are a major issue also. And especially - any custom code. I try to keep mine short, clean, safe.
I had the same experience on my localhost. It ran slow but my machine is not a bear, more like, koala or lemur. So between what I learned, Esmi's input, and having this local setup that did not reset ever, I said this is the wall and it's time to look for another service. With justhost, it's faster on the server than ever. No glitches. The one issue I did have was justhost's security. Their service is not easy or intuitive to set up due to that, but their help is good and once this is done, you're in, end of story. I'm not plugging their service or anything. Do some homework. There are several in any price range that offer no limit to pages. Due to variance in what we're dealing with, no host can offer nirvana, just what works for your particular setup.
Multiple themes: There are several plug-ins that say they do this cleanly. I tried a few and found the jonradio-multiple-themes to work for me. I don't know how it will work as well for anyone else due to the fact I am switching between half a dozen very similar themes, all modified from the original, as opposed to totally different themes, possibly making theme switching easier for the plug-in. Know that theme switching is a touchy process that is difficult for WordPress, so any plug-in that claims to do this has to be cleanly written and possibly for a specific set or type of theme set. Add to the pie that themes also are varied. Add any plug-ins, widgets, and forbid - as i live on - custom code, and ------ Whew!
The lesson learned here is that not all providers of any service are equal. Some are better at some things than others while others are better at some things than some. I still have sites and domains with GoDaddy with no intention or need to move them. I Can tell you that Both these service have knowledgeable and nice folks on the phone to help you. That's a plus. Short wait times, pleasant crew, easy to understand, and they know when to get tier II or III. Who wins? The customer!