LAN / WAN issues when transferring large amounts of data

I'm having an issue when I copy data from a PC to my NAS at gigabit speeds. My Router is an Asus RT-68U

ROUTER --> Port 3 --> Printer
    |
    |
    | ---> Port 1 --> NetGear GS108PE Switch
                              |
                    VLAN10    | --> Port 1 --> NAS Synology (1000)
                    VLAN10    | --> Port 2 
                    VLAN1     | --> Port 3 --> PC (1000)
                    VLAN20    | --> Port 4 --> IP Handset (10/100)
                    VLAN20    | --> Port 5 --> IP Server
                    VLAN10    | --> Port 6 --> NAS Zyxel (1000) 
                    VLAN10    | --> Port 7 --> Link to Netgear FS116 --|
                    VLAN1     |---> Port 8 Link to Router              |
                                                                       |--> Ports 1 - 16 10/100 devices


VLAN-ID Port Members
1         1 2 3 4 5 6 7 8
10        1 2 3       7 8
20            3 4 5     8

Devices in VLAN10 can talk to each other and the Internet, but not VLAN20

Devices in VLAN20 can talk to each other and the Internet, but not VLAN10

The PC in VLAN1 can talk to anything

The issue is when I copy GB's worth of data from the PC to the Synology NAS I get full gigabit speeds but all devices lose the internet.

I'm assuming this is due to the PC being in VLAN1 the same as the routes default VLAN and the large transfer is killing the router. I get a similar issue if my laptop via Wifi to the router attempts a large backup to the synology NAS.

Any way to over come this ? I though the GS108PE switch would only route the data from port 3 to port 1, so I'm confused as to how it's affecting the router.

The GS108PE doesn't have any QoS, Rate Limit or Broadcast Filtering configured.

I have tried using port based VLANS and have set the port assignment as above, but then I could ping devices across the VLANS so I'm not sure whats happened there. I've also created a new VLAN 5 to replace VLAN 1 incase that was causing the issue.

Is there any way to do this ? as I'd like to be able to do large data transfers between local devices whilst using the internet.

Thanks