Verizon DSL is the worst around here because (I agree) DSL should not be used for business. I've seen roadrunner go down to 25 kbps upload for very short periods of time. I know this sounds crazy, but the speedtests confirm it. There have been a lot more than one call where I get sent in to confirm the internet connection is bad and the ATA is fine. It pays well at least and the work is easy.... tell ISP tech support to send a line man or boost the voltage.
Sounds like you have a VPN figured out. I've not worked with comcast too much so really can't comment on them.
When bandwidth goes down (usually the uplink) we always see the transmitting voip only broadcasting every third or fourth word. The result is choppy audio to say the least.
I'm curious... What device are you using for 8 lines? Or is it multiple ATAs? I'd heard there were a few newer ones coming out at a reasonable price, but haven't played with them yet.... I'm daisy chaining the 4 port jobbies still.