Fatal Errors in WP, but they go away on IIS restart

Hello,

I’ve been recently having issues with one of two WP blogs I have. Both are on same server, one is with a bit low traffic and one with a bit high traffic (~100 visits / day).

The server is IIS 7.5 with PHP 5.5.11 (installed with MS WebPI).
Edit: This is on Win2008 R2 VPS.

The blog with high traffic works good for few days, and then randomly shows blank page to visitors, wp-admin continues to work.

The PHP error log file have these errors:

PHP Fatal error:  Call to undefined function wp() in [blog-root]\wp-blog-header.php on line 14
PHP Fatal error:  Call to undefined function load_textdomain() in [blog-root]\wp-admin\includes\admin.php on line 15

When IIS is restarted, the blog starts working again as usual. I’m no good PHP guy, so really not sure whats going on and unable to debug myself.

Is it a memory issue? maybe there’s some php.ini settings to fix it?

Any help would be highly appreciated. Thanks,

// chall3ng3r //