Hi,
Here is what I think the problem is...
(I experienced the same phenominum.)
With A8 and the new PhyX system of collision, the terrain NEEDS to be a scale of 1-to-1 in WED.....
In other words... You can't scale the terrain in WED!
You will need to scale the terrain in MED and then put it into WED and do not scale it in WED...
I'm not sure if you scaled the terrain in WED, but it sounds just like the problem that I had to figure out...
But... if the problem isn't from scaling, look into Tobias's suggestions...
Best of luck!