This *could* be the beginning of a hardware failure. Loss of network connectivity will do what you're describing, and the reset (battery pull) restoring it tends to back this up. Another possibility is a loose battery. You might try cleaning the battery contacts with a pencil eraser, both on the BB and the battery. You can also try slipping a piece of paper on the end of the battery opposite the contacts to try to hold it tighter to the contacts. That's called "shimming" your battery. Loose batteries have caused a ton of problems folks normally wouldn't associate with such a seemingly trivial problem.

If none of that works, before writing if off completely as bad hardware you might try updating/re-installing the operating system: How do I upgrade the OS on my BlackBerry? - BlackBerryFAQ