In the "Guideling to setting up a functional VPN" you will find references on how to test the MTU. I'd recommend that you do that. (
http://www.wingate.com/files/VPN_Setup_Guide.pdf)
The VPN has a header that it needs to attach to every packet. If the MTU is too close you could be experiencing a fragmentation problem; where the header added puts it over the MTU, which will result in every packet being fragmented, etc. etc.
If you do the MTU tests as described in that document (Basically consists of pinging with a specified packet size and no fragmentation until the packets cannot be sent through) you can work out what the best sized MTU should be to take care of the VPN headers.
When you are pinging a machine across the network, what time do you get? E.g.
pinging router wrote:Pinging 192.168.13.1 with 32 bytes of data:
Reply from 192.168.13.1: bytes=32 time<1ms TTL=64
Reply from 192.168.13.1: bytes=32 time<1ms TTL=64
Reply from 192.168.13.1: bytes=32 time<1ms TTL=64
Reply from 192.168.13.1: bytes=32 time<1ms TTL=64
Lastly, 6.0.3 is significantly better than 5.2.3. Especially wrt the driver. You can use your 5.2.3 license key with 6.0.3.