Hi - this same exact thing is happening to me. Just had the install done today. I have the SB8200 and the speed test is very unstable. I will watch it spike to 700, then wonder all over the place until it settles on what seems to be an average of around 500ish. Using XFinity speed test on all of this. The other thing that happens is periodically my speed average will drop to even 60Mb. That's never happened before on my DOCSIS 3.0 gateway. It would run 180Mb/sec down 100% of the time when I tested to my local pop. I live in Windham NH and am using the Foxboro location as test. Latency is consistent at 10-12 ms.