This is probably what you are referring to with the speed tests. Just make sure you do the tests with a single device connected directly to the modem with NO ROUTER.
Just a question. Is this happening with multiple devices, or is it happening with a single device that you use?
It's not likely that a 0.5 volt difference would be causing the problems. Normally components are pretty reasonable with small drops or raises in voltage, and one of 0.5 volts isn't likely to trigger anything.
The reason I asked about what devices were having the problems is that I was thinking that it could be a network card issue, but if you are having issues at the same time with other devices it's likely not what I was thinking it could be.