RE: buffering, your point about it being unacceptable to the callers is correct. VoIP endpoints do have small (and adjustable) buffers, called jitter buffers, but they're only designed to mitigate very short signal dropouts. Buffering any more than about a quarter-second becomes noticeable and disconcerting to the callers, since it creates a delay that that messes with the brain's ability to follow the conversation.
This is a tricky problem to isolate. You can't tell anything by looking at download speed, since the speed measurements are averaged over time. As for the power supply, you can't tell much with a voltmeter, other than it being completely dead or not. You'd need to connect an oscilloscope to the power line to watch for distortion or brief dropouts in the voltage, and/or harmonics, caused by some component or electrical connection in the switching power supply starting to fail.
You've already eliminated the service provider (Anveo or
voip.ms) as the cause (because it happens with both of them), and you've eliminated the configuration as the cause, because you re-did it.
You could now further isolate this with these two tests:
Use the SIP VoIP quality test at this website, which uses a Java applet to simulate an end-to-end VoIP "conversation", and measures multiple parameters to come up with a MOS (telco industry term for "Mean Opinion Score").
http://myspeed.visualware.com/index.phpRepeat the test multiple times, to a few different endpoints, and if you score less than 4.0, you have an internet service issue. It's really common for cable internet to exhibit these issues, caused by some sort of corrosion or water incursion on the coax cables or connectors. Occasionally, the cable modems go bad, too.
If you consistently score 4.0 or better (4.2 is the max), then, an easy way to isolate the OBi hardware as the culprit would be to unplug it, and install a SIP VoIP softphone on your computer. Counterpath X-Lite is free and very easy to set up, but there are others out there. If calls never disconnect while using the service via the softphone, then you've got a hardware issue (Ethernet cable, power supply and/or OBi box).