I have also been experiencing a similar issue. I have a MI424-WR modem and it has been working well for many years. No wiring changes in the house, no coax changes, no physical changes in the location of the modem.
About 3-4 months ago (March 2013), I started to notice that my VPN would disconnect. I thought it must be something odd with my VPN software, perhaps a setting that caused the VPN to drop every so often and then automatically renew.
But then about 2 months ago (April 2013), my VOIP phone started to drop, right in the middle of a call. I would have to hang up and redial the person that I was speaking to. That's when I started to suspect that perhaps my internet connectivity was dropping.
I opened up a ticket with Verizon a few days ago but the technician chased after speed issues (download speed, upload speed), and didn't address the actual challenge which seems to be intermittent drops. We then got disconnected (ironic) and the tech didn't call back.
Model: MI424-WR
Firmware version: 4.0.16.1.56.0.10.14.4
Hardware version: D
I intend to open up another call with Verizon and ask to have the modem swapped out.
I have viewed the log (System Monitoring > Advanced Status > System Logging), but I don't know how to decipher the log entries and determine if any of them are meaningful or point to a root cause.