unity 5 Speedtree Judder/tracking lag/double image

Hi all, we are currently facing a show stopping problem using Unity 5 where we are getting extremely bad double image and juddering, along with tracking just pausing for 1 or 2 seconds altogether, although the mirrored images on the monitor are perfectly fine when using the mouse to look around.

through trial and error it seems to be only when i enable trees on my terrain. here is our setup and some further information on the troubleshooting i have performed.

Unity version 5.0.0f4
Windows 7 64 bit
Doing 64 bit builds of our project
Same result on many machines, all extremely high spec (Nvidia 980 video cards, i7 CPUs, 16gb ram)

We are using speedtree models and i have tried them both with and without smooth lod enabled. Have been very aggressive with the LOD and culling settings, and also tried reducing draw culling distance down to 100 or even less for testing. I have also disabled colliders on the trees.

The Scene:
Our scene has 4 x terrains of about 13000 x 13000 each
At first i though the problem was the terrains as i would disable each one, one at a time and do a build to test.. this improved performance until with no terrains it runs perfectly smoothly.

in later testing i disabled the trees one terrain at a time and had the same results, so it seems the trees are the culprit, ie: with all terrains, but no trees, performance is great.

Even though there aren't lots of trees being drawn for example with reduced far clip distance set in OVR camera rig down to 300 with only 1 tree visible, the issue still occurs, although to a lesser extent.. the more trees, the worse the issue it seems.

Has anyone experienced anything similar? this is extremely worrying as we have project delivery in a few weeks and we have no idea why this is happening.

thanks for any help in advance.