iPhone Messages SMS app problem

Ive been having a problem with the Messages (SMS) app on my iPhone 3GS. As a matter of personal choice (or rather by nature since Im slightly prone to OCD), I like to close (or kill) all my apps from the multitasking bar and have it stay nice and clean every time I use my iPhone. However, Ive been getting lazy over the past few weeks and so I let the messages app stay open or on the multitasking bar so it opens quickly. The problem Im having is that whenever I close (or kill) the Messages (SMS) app, (tapping on that little red sign when the apps are wiggling on the multitasking bar), my iPhone freezes for a while (anywhere from 10 seconds to minutes). This time is directly proportional to how long I leave the Messages app open for, and how many messages I receive/send. Sometimes, it does a hard reset on me by crashing completely to the Apple logo.
Now Im not sure why this is, but my uneducated guess is that it has to do with the messages that I send/receive being indexed for search or something. Since the longer the app stays open, the more messages it has to index, it overwhelms the iPhone and sometimes crashes, and almost always freezes.
But, Im not an expert by any means. So can anyone explain to me what this is/how to fix it (if there are any solutions at all)?
Thank you in advance!
iPhone 3GS iOS 4 Running whatever the latest version of iOS is