vBulletin Process spikes causing errors

[5starpool]

My forum is hosted on a hostgator shared server, and has a process limit of 25 concurrent processes. Most of the time there is no issue, and between 2-4 processes are running at a time. Occasionally though, the processes will spike, causing error 500, and making the site unavailable. Here is a sample list of processes running at a time when my site is unavailable.

32572 /usr/local/cpanel/cpanel ./frontend/x3/hg/processes.php 0% 0.1% Kill Process
32185 /usr/bin/php/home/cpname/public_html/forum/editpost.php 0% 0.1% Kill Process
32169 /usr/bin/php/home/cpname/public_html/forum/index.php 0% 0.1% Kill Process
32099 /usr/bin/php/home/cpname/public_html/forum/index.php 0% 0.1% Kill Process
31900 /usr/bin/php/home/cpname/public_html/forum/editpost.php 0% 0.1% Kill Process
31881 /usr/bin/php/home/cpname/public_html/forum/index.php 0% 0.1% Kill Process
31846 /usr/bin/php/home/cpname/public_html/forum/index.php 0% 0.1% Kill Process
31839 /usr/bin/php/home/cpname/public_html/forum/index.php 0% 0.1% Kill Process
31702 /usr/bin/php/home/cpname/public_html/forum/forumdisplay.php 0% 0.1% Kill Process
31546 /usr/bin/php/home/cpname/public_html/forum/index.php 0% 0.1% Kill Process
31534 /usr/bin/php/home/cpname/public_html/forum/post_thanks.php 0% 0.1% Kill Process
31506 /usr/bin/php/home/cpname/public_html/forum/showthread.php 0.1% 0.1% Kill Process
31445 /usr/bin/php/home/cpname/public_html/forum/index.php 0% 0.1% Kill Process
31296 /usr/bin/php/home/cpname/public_html/forum/misc.php 0% 0.1% Kill Process
32178 /usr/bin/php/home/cpname/public_html/forum/index.php 0% 0.1% Kill Process
31728 /usr/bin/php/home/cpname/public_html/forum/index.php 0% 0.1% Kill Process
31647 /usr/bin/php/home/cpname/public_html/forum/forumdisplay.php 0% 0.1% Kill Process
31636 /usr/bin/php/home/cpname/public_html/forum/ajax.php 0% 0.1% Kill Process
31538 /usr/bin/php/home/cpname/public_html/forum/index.php 0% 0.1% Kill Process
31380 /usr/bin/php/home/cpname/public_html/forum/showthread.php 0.1% 0.1% Kill Process
31300 /usr/bin/php/home/cpname/public_html/forum/misc.php 0% 0.1% Kill Process
31297 /usr/bin/php/home/cpname/public_html/forum/misc.php 0% 0.1% Kill Process
31229 /usr/bin/php/home/cpname/public_html/forum/showthread.php 0% 0.1% Kill Process
29553 /usr/bin/php/home/cpname/public_html/forum/misc.php 0% 0.1% Kill Process
27105 /usr/bin/php/home/cpname/public_html/forum/misc.php 0% 0.1% Kill Process

As you can see, most of these are standard vbulletin php files that would be accessed thousands of times a day, and I would imagine would only last a fraction of a second usually, but only occasionally does it spike. This is not caused by a surge in users, as it is pretty constant usage at around 80-140 users for most of the day (the rest is quieter). I have run the vB optimisation things mentioned in this page :http://www.vbulletin-faq.com/optimiz...tin-server.htm but it still occurs intermittently, and I have no idea why. I have talked this over with live support on hostgator and they are adamant that it is nothing on their side that is causing this.

Could it be caused by vb hacks I've installed? I haven't installed any over the last few months that I can remember, and this has mostly been happening the last 2-3 months. My forum is about 17 months old and has around 340k posts. There is also one very large thread of 150k posts. Could this have anything to do with the issue?

I am at a loss here I'm afraid, so any help would be greatly appreciated.