NEVER upgrade your version of PHP unless you absolutely have to. Two weeks ago, the hosting company 'suggested' that I upgrade all of my domains to a newer version of PHP (v5.3.x), so without thinking, I clicked 'upgrade'. That proved to be a big mistake. The problem is, I'm using a redirected, alternate, 'cloned' php.ini file which increases the upload_max_filesize, post_max_size and memory_limit variables. Increasing those limits allows larger files to be uploaded through the forum. The problem is, the script I used to redirect the PHP server to my revised file copied the original php.ini file and when I upgraded the version of PHP, my revised file (being older) does not comply with the parameters of the newest version, so everything came to a screeching halt.
On top of that, SMF doesn't like PHP v5.3.x - I was getting all sorts of random parsing errors that I never seen before the upgrade. After doing some research and asking other webmasters, they suggested I skip PHP 5.3.x and try 5.4.x. If the fixes I made don't work, I may go that route.
I rolled back the version of PHP, but I'm still getting random freezes and timeouts, much like a year ago when Dreamhost was going through its infamous server reliability issues. Now, after a year of nearly trouble free, (mostly) reliable and fast server speeds, I'm back to nothing but problems.
The moral of the story is that - if it ain't broke, don't fix it!