Sit to Site gate way keepsing dropping in and out

I have set up a site to site link from azure VN to local network using a router. I see it stats it has succeeded from the Azure portal, but after about 15 to 30 seconds it then says not connected , 15 seconds later it says succeeded again then not connected it just keeps doing this. 
Is this a symptom of something, any ideas.
Thanks

Alan Mosley - ThatsIT Solutions