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Okay. Here's what my Tigertech guys found:

As a followup to this: We've looked into it in detail.

When we first logged in, there wasn't a deactivated "Simple:Press" plugin -- perhaps you forgot to re-add it? -- so we downloaded the demo one from their site and installed it.

After doing so, we did see intermittent 500 errors, as you said. These all left an extremely unusual error in the logs:

PHP crashed on opline 8 of classes() at /mysitepath/wp-content/themes/dejavu/framework.php:119

Some experimentation indicates that something about the SimplePress plugin causes PHP to crash when eAccelerator is enabled (and perhaps only when ionCube Loader is also enabled, based on some additional testing, although we couldn't confirm that because we can't fully disable ionCube Loader on [the site]).

This is the first eAccelerator/script incompatibility we've seen in some years, so we're surprised by this. Some searching suggests that this can happen when software does many different levels of "require" and "include" in PHP, possibly because it's running out of memory.

For most customers, we'd recommend switching to PHP 5.4, which contains
updated versions of both ionCube and eAccelerator, and our guess is that it would probably solve it. However, in your case, you have a very fragile mixture of PHP versions already running, and we're reluctant to touch it, which means that the safest thing to do is switch off eAccelerator, at least for now.

Doing so appears to have solved the problem. We've left the Simple:Press plugin enabled, but we no longer see any 500 errors. Let us know if you agree that it's solved.

Disabling eAccelerator will make the site slightly slower, though. You'll need to test it and decide whether it's still okay. (We didn't notice it feeling slower after we made the change, but we don't really have a feel for the "normal" speed of it.)

If you ever stop using this plugin, or when you can eventually upgrade to PHP 5.4 or later (we'll soon make version 5.5 available), you should re-enable eAccelerator in our "My Account" control panel and see if it works -- we guess it will.

Please let us know if there is anything else we can do. Thank you again!

--
Robert Mathews, Tiger Technologies

It's one of the subversions---but I'm stuck there because I'm using one-off software that can't be upgraded.

Yellow Swordfish said
Fascinating.... I don't know which version of php you on of course but one of them - and I think it was php 5.2 - was notoriously flaky and is not recommended t be used on a live site. Although it might have been a sub-version of that.

It's good of your host to do so much in the way of diagnostics. Do you plan on accepting their advice?

Absolutely. I have the best host ever (after having spent years with many that were pretty bad). And the dev site I'm building does not have the php-version-dependent weaknesses of my live site. The second we get this sucker live, they'll bump me up to the current version of php.