Sorry that wasn't any help for you, but now that I've removed ip virtual-reassembly from all interfaces, the connection has been rock solid, even with less than optimal signal. I've only got 2-3 bars at my desk. I'm taking it home with me this weekend to see how it performs in the sticks.
I'm strictly using VPN, so there's no way for the carrier to see any RFC 1918 addresses; only the ISAKMP and IPSEC between the endpoints of the tunnel are hitting the carrier network.
I've tried all these solutions and nothing is working. I'm beginning to be convinced that IPSEC and Vorizen 3G aren't that compatable. Does anybody have a good article on setting up GRE? I have a 2811 with advanced security I can set up here since I've only got 10 sites connected 3G.
This article is extensive but should have all the info you need.
The 2811 with Advanced Security is good for what you want to do and is capable of making these type of connections
Just an update to this situation for anyone who may be experiencing the same issues. I talked with the Verizon network and they said that the line is experiencing an IP Source Violation. I am going to be setting up a monitor with them tonight to see if the IP in issue will show itself. If I get a fix on this I will post it.