My copy of the 2008 Gitzo catalog shows the Series 3 tripods suitable for lenses up to 300 mm focal length. Series 4 tripods are rated for 400 mm, and Series 5 for 500 mm. The chart does not extend to 600 mm focal length. These limits are a function of torsional rigidity.
The Nikon 300 mm lens weighs 6.4 pounds, and the 600 mm F/4.0 weighs 11.2 pounds. Add the weight of the D3S camera (2.75 pounds) and the RRS pano system to the 600 mm lens, and your are probably at something over sixteen pounds or so, which is well within the maximum load limits for the Gitzo Series 3 tripods: 26.4 pounds for aluminum legs, and 39.6 pounds for the carbon fiber legs.
As mentioned above, torsional rigidity may be an issue here, particularly if the tripod is extended to its full height. Is the tripod a three or four section leg model? The 600 mm lens has a field of view (FX format) of 4.17 degrees. If you can examine the original image above to see if the apparent movement is lateral, we may have narrowed the issue down to possible torsional movement.
Before you purchase another heavier tripod, can you repeat the test with the configuration on a concrete slab? Then capture a series of identical images (same subject and subject to lens distance, etc.) with the tripod legs at different extensions, from the shortest, collapsed configuration, to fully extended. Start with all sections fully extended, then collapse them one at a time until they are all collapsed. Examine the images at 100 percent (actual pixels) on screen and see if you can identify the configation that begins to show image blur. And, is it blurred in roll, pitch, or yaw? (It may be difficult to distinguish between roll and yaw.)
Is your location subject to heavy truck, bus, or other vehicle traffic? Our house backs up to a fairly busy arterial road and I can feel the vibration in the floor when a heavy truck goes thundering by.
If this exercise is inconclusive, we may have to look at focus issues. I assume that this issue has occurred with the 600 mm lens only, correct?
HBB in Phoenix, Arizona Nikonian Team Member
Photography is a journey with no conceivable destination.