I would say that Freddie is most likely correct. That is what I would have guessed. However, I did have some trouble similar to this a long while ago. Its probably not be the same as yours though. A while back I was working on a long wooden coaster (~2 1/2 minutes of track after lift-hill). Towards the end of the ride, g-forces would vary greatly.
Perhaps you could post some screen shots? What version of NoLimits do you have?