I also had the same problem for some time.
Today, I removed the silicone skin that I have on mine, and all of a sudden, my issues with email fetching/pushing are gone. Yea, sounds really far-fetched, but so far I have had minimal (if any issues) with getting emails.
I Still have issues with maintaining a 3G connection, but at least I'm getting email transfers, whereas before I too was needing to reboot as well.