I have the Space Traveller, and I had similar problems. It would start as soon as I added the fourth or fifth component to the assembly. I found that it was the center of rotation setting. I think that I had it set on Center of Volume, and when I switched it to Center of Mass the problem went away. If neither of those settings work, try selecting a new point and setting it manually. Post back if that fixes the problem.
Good Luck.
Myles