Packet loss between Netgear GS748T and Draytek Vigor 2860

Hello

Hoping someone can point us in the right direction after pulling my hair out for the last 2 days on this one.

We have recently installed 2 Netgear GS748T switches (single link connection between the two) into a client's site (approx 30 users but site wanted all wall ports patching in). Since then they keep getting random disconnects on the internet line, but not all at the same time though.

We ran some ping tests through the network - all pings to anything on the internal network is fine, no drops, even after several hours. However pings to the router itself (and internet) are showing sporadic packet drops (anything from 2-7 drops per 10 minutes).

The Draytek uses 2 VLANs - 1 for data (which is connected to the Netgear switches), and another for the phone system (which also seems to be having issues as a result of this too which lead us to suspect the router may be faulty.

We connected to a spare port on the router on the data vlan and confirmed still having the issue when directly connected. So we swapped out the router and the network cable - still having same issues.

I've tried disabling STP, no change. DOS defense is not enabled on either system, we are using VLANs and all of these are setup correctly, no QoS in use either.

The router is on the latest firmware, the switches are on firmware 6.3.1.11.

At present I'm not sure what else to look at or try - there's obviously something happening between the switch and router but for the life of me can't work out what.

Anyone have any helpful suggestions on this? Anyone come across this before?

Thanks in advance.