Thanks for this, i didnt twig and it fixed my issues.

Symptoms were

BT ADSL-Wan1
Virgin media - Wan2
Load balanced between.

Local VPN clients runing over UDP =- 10secs traffic then pause and recover.
packet trace shows lost packets and recovery.
VPN via TCP was fine.

Turned off Firewall >> DoS defense Setup

Enable UDP flood defense = off

problem gone. This was also showing its self up in various ways due to the 150 packets / sec limit. web pages half loading and random pauses.

I disabled the UDP defense and things have been fine for 6 days but it did drop out today.
I've now written a script that pings Google every 5 minutes and if it can't find it then it sends a reboot command to the router and that seems to have sorted things.
Mike

Have you tried setting the PCs to use an alternative DNS server?

Such as Google's -
8.8.8.8
8.8.4.4

Our PCs use SME server for their DNS and I believe that it gets its DNS from its own list of servers.
Mike