Thanks for the reply. Actually, since I posted my earlier comment I went ahead and moved a couple of sites that I was having this issue with to Greengeeks.com and the sites load WAY faster now with no such issues. There are apparently key php caching features such as memcache that are very useful for Drupal, but are not being used at Rackspace Cloud Sites and the lack of these slow things down considerably.

I really like Rackspace overall, and will encourage them to make some changes to their service, and in the mean time, some of my clients' sites just won't be able to be kept there.