Well I did have FS bridging to an asterisk box behind it, but wanted to check it wasn't the FS box which was the problem to did a simple dial plan with some music playing back.
The interesting thing is, I used one of the latest GIT versions and managed to get 1000 calls, but I would expect with the hardware i have to get even more.
I would rather use a stable version of the software, it's just bizare that newer hardware with much higher spec doesn't perform as well.