My broadband connection is a Virgin Media Businss 500/50 Mb/s service with 5 static IP addresses.
The mainstay router I have used for some time is a Vigor 2960, which has been fine until recently, but has been constantly rebooting now. Support are trying to help find the reason.

I also have a spare 2862, which I have been using while troubleshooting the 2960. What I have found is that when I am using the 2960 (and it's not busy rebooting!) Youtube works very smoothly. Switching to the 2862 I get a lot of buffering on Youtube, especially if I move the playback position to further into the video.

This is with both the 2960 and 2862 connected to the same Virgin Media business connection. I've even tried swappingthe public IP address each uses, but the results are still the same - good YouTube performance on the 2960, less good on the 2862. Clearly a router issue not a broadband connection issue.

I have been considering a 3910 to replace the 2960 if it proves to be a hardware fault, however concerned at this difference, and wonder if it is linked to the 2960 being linux based and the 2862 DrayOS. I am aware that the 3910 is also DrayOS - so likely needs a reboot after many config changes. A reboot is somethign the 2960 never needs with any config changes.

I'd be interested to know if others have expericend a buffering issue, and how the 3910 performs overall.

There could be a few things. #1 is the different default MTU - looks like 1500 is the standard MTU on the 2960, while the Vigor 2862 should be 1492. Putting that up to 1500 (and maybe disabling the Change TTL setting) could help.
Beyond that, the two routers have different architectures for acceleration, which I think you'll need to have enabled on the Vigor 2862 to get the full speed of that line.
Go through the Troubleshooting > Performance steps of this guide for all the things on the Vigor 2862 required to get Hardware Acceleration working for both up and downstream: https://www.draytek.co.uk/support/guides/kb-fttp-wan-setup

I think that if moving around in a Youtube video makes a new session to do that, the Vigor 2960 would be quicker potentially because of its different method for accelerating traffic. The Vigor 2862's hardware accelerator is good but it takes a second or two to go from establishing the session through the firewall (slow part) to put it through to the hardware accelerator.

Or it could be something more fundamental, like some problem with QUIC (assuming you're using Chrome) or a DNS problem somewhere.

Probably the MTU on the WAN side, as said.

Great points about the WAN MTU. With Virgin media business, since they use a GRE tunnel to facilitate the static IP routed subnet option, the MTU is 1460. This is set both on the 2960 & 2862. I also used the 2862's MTU dscovery option to verify and it confirms 1460 to be correct.

You are right on the 2862 without Hardware acceleration a speedtest maxes out at about 375 Mb/s but with HW acceleration set to auto the 2862 achieves full line speed on a speedtest.
Thank you for the link to the guide - I'll read through that to see if this will help.

Ultimately this 2862 isn't the router I planned to keep for this, and have been considering a 3910. Given the significant power of the 3910 II would hope it will not suffer this bufering issue when moving playback position.

It does show what a great device the 2960 is though. If we can get to the bottom of these constant reboots I'll keep it in place for now.

My 2860 has always made Facebook and Instagram videos buffer and freeze. No other video streaming was affected.

After a LOT of research I finally found the problem - it was the UDP flood defense setting in firewall defense setup.

Increasing the packets / sec threshold from 2000 to 4000 solved the problem.

No idea why.

Thank you for this - I've just spent some time testing this on my 2862 and the difference is significant. Having increased that value to 4000 I get no lags and buffering. At the default 2000 I get a lot.