White screen of death on 2 sites of multi-site install

BEFORE YOU READ: THIS LOOKS FIXED, THANKS TO KENORB'S POST BELOW.

Thought I'd disabled the theme (cti_flex; seems issue in this, tho had been working well till now), by renaming it, setting its status to 0 in mysql table. But, deleted it, and sites started working again.
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SO MAYBE DON'T READ THIS; OR SKIM THRO TO SEE HOW IRRITATING WSOD CAN BE! (and how I'm, err, definitely no redhot techie)

I have a multi-site install of Drupal 6.14: 10 sites.
Was ok till last night, when - after not doing much to the sites for around three weeks - I visited one of the two main sites, to edit an article. Found I could access home page, but not the article: article page was empty. Pages cached with Boost were still ok.
Within half an hour to an hour, though, I "lost" home page, and no pages cached by Boost either (after its cache was emptied; no new pages added to cache).

So, White Screen of Death for a site: even admin. [On Firefox and Opera, that is; with Safari, get message saying "can’t open the page ... because the server unexpectedly dropped the connection."
I also visited some of other sites. One of these soon suffered same problem: WSOD throughout, even admin.

These were the two key sites, by far the largest I have.

Yet, other sites still running okay; one of them using much the same set of modules, inc Boost.

As well as re-activating sites after leaving them pretty near dormant, server has changed as host upgraded to FreeBSD 7.2. Googling, looks like this should not be an issue.

I have tried searching, read and tried working through the White Screen of Death and Silent WSOD articles at:
http://drupal.org/node/158043 and http://drupal.org/node/482956

- but as yet, no solution.

Contacted webhost: cron jobs look to be running okay today; so evidently not an issue with a "stuck" cron job as I found on another thread.
I wondered re cron; was running with poormanscron, so this might not run so often when most pages cached and few registered users. Since last night, I've removed poormanscron, and using a crontab for hourly cron running.

Have seen sites almost come back to life, a couple of hours ago: only partial, but exciting to me at the time; they vanished again.
One reported the Boost crawler was still running, which had me wonder if this might be problem: I have left sites, so many pages will have been cached by Boost; making changes led to pages needing re-caching, also started crawler again.

Some of things I've done:

- Enabled error reporting. Yet, even with this, pages invariably just blank.
- Added dtools. Even the link to wsod-emergency.php tending to give blank screen. Has given brief reports re php_exit errors; but these don't seem so severe.

Looked at watchdog files, in mysql table.
Getting several saying "page not found" - even for home page.

Cleared cache table; also other cache tables like cache_menu

Worked on one site, in particular; changes included:

amended php timeout, to infinite (in settings.php)
also in settings.php, massive increase in memory for php - from 42M, which had worked well for months, to 64M, 128M, even 384M (on shared hosting, so not sure if will be able to use all this memory...)

Disabled contributed modules, inc by renaming all/modules folder

Disabled Drupal modules, by renaming modules folder (yes, of course didn't work; saw a little life though, with error messages)

Created new folders for css and js, giving 777 permissions.

I don't write code; don't have custom snippets I can recall.
Nothing re Zend optimizer that I know of.

Both sites - unlike others I have - run with cti_flex theme; had been running well.
I've tried disabling this theme, via mysql (setting status to 0, from 1) - but no benefit.

Other main difference I can think of with the sites having problems is size: substantially larger than other sites; so a lot more pages in each.

I use pathauto. Haven't include the line for include paths in htaccess; not fully sure what to write for my case, and doesn't seem this should be needed: I haven't changed host.

Anyway, time for me to get away from the computer I think!
I'd greatly appreciate advice.

Martin