Site speedy after migration but slow after one day

Hello,

I have a Drupal site running at the following location (levelground.com). It was completed and made live around the end of September. In the last month the site has been increasingly having speed problems, often not even being able to display at all (could not connect to server errors).

I've since switched the site to DreamHost and for the first day it ran really quick but then started slowing down. So slow that I wasn't able to get onto the site to access the admin interface.

After that, I updated all the modules and drupal core to their most current versions and things seemed to be working fine again. Now, 24 hours later, I'm back to a site that isn't able to display (500 internal server error).

I feel like there's a script that's looping or a process that isn't quitting and that's what's causing the problem. Any suggestions to help me troubleshoot the site would be welcomed.

Here's the basic setup:
Drupal 5.15
Php 5.2.6

Modules (latest versions as of today's date):
active_template
blockcache
calendar
cck
cck_address
contemplate
date
devel
fckeditor
image
imageapi
imagecache
imagefield
imagefield_gallery
imce
jquery_update
mimemail
securepages
simplenews
simplenews_template
store_locator
tapir
token
ubercart
ubrowser
uc_canadapost
uc_fedex
views
wholesale_form
workflow_ng

Thanks in advance for your help.
Rich