What version of Windows is this? It sounds like when the window is out of focus, it is put in a hold state. There may be some Win32 settings you can use to let it continue to run in such a situation, but I am not expert enough with Win32 API's to tell you how to do that.
For Each ctrl As Control In Me.Controls("pnlMainPanel").Controls
If ctrl.GetType Is GetType(System.Windows.Forms.Panel) Then
For Each subCtrl As Control In ctrl.Controls
If subCtrl.GetType Is GetType(System.Windows.Forms.TextBox) Then
If subCtrl.GetType Is ...