Mail.app randomly stops checking for new mail

[shigs]

The title pretty much says it all. From time to time the iphone mail app will stop checking for email. Even if I restart the phone it will not always start checking again. As for my phone, I have an iPhone 4 4.0.1 jail broken and it is set to auto check all 3 of my email accounts every 15 minutes. A few minutes ago (9PM) I noticed I had email on my computer but I had no notification on my phone so I opened my mail app and it says "Last Checked 10:45am". When I open the mail app it does go ahead and check manually and downloads all the mail as it should. This has been happening for some time, I did try doing a restore and re-jailbreaking the phone but the problem persisted. The only idea I have is to restore, re-jailbreak, and monitor how the phone runs as I install each app but since this happens at random this could take quite a long time. Can anyone offer any advice? Thanks in advance.