With Android, you can't freeze background apps and you shouldn't even try. It's not like the BB system where you have to make sure your apps are closed or risk battery drain. This may be your problem.
Are you running Task Killer? Get rid of it asap! It's bad news for Android systems. The X - and other Androids - are designed to keep apps running for a while after you are done using them. Then they are closed automatically. This is done to conserve battery power that would be used to re-open apps. Task Killer is a killer for the X. TK closes an app prematurely and the X re-opens it in the background. There are lots of threads in the forums about the negative aspects of Task Killer and the counter-productive results of closing apps yourself. Not needed.