There are many reports from users in your area and on your carrier that experience the same. Some have escalated through their mobile service provider and been told it is network congestion, but the "blame" shifts back and forth between the carrier(s) and RIM.
To get more attention to the matter, I recommend you ring up your mobile service provider and lodge a formal support ticket. The more of those that are opened, the more likely the issue shall receive the attention it requires.
Occam's Razor nearly always applies when troubleshooting technology issues!
If anyone has been helpful to you, please show your appreciation by clicking the button inside of their post. Please click here and read, along with the threads to which it links, for helpful information to guide you as you proceed. I always recommend that you treat your BlackBerry like any other computing device, including using a regular backup schedule...click here for an article with instructions.