SMS messages not showing up right away

Here's my problem:

Every little while, SMS message are not coming through on my phone, unless I wake up the phone, unlock it (S2U2), then only one message comes through. All next messages will only come in after I exit or minimize the messaging application, and then one more will come in. Always only one by one, so I have to keep on minimizing the messaging application.

Really annoying... sometimes rebooting the phone will work, but not always.

I also have Mobile Shell installed. I'm not sure if this problem is related to this or other software, or a problem with the phone itself or its pre-installed software...