It is most likely 1/60th, but it could be less common than that. For example, one of the stadiums I've been to has its lights on two different power grids, for reasons related to how their power got brought in rather than things we might think of. (BTW this is very common for data centers, which are normally attached to - at least - two different grids for redundancy reasons.) In the case of this stadium, the dips only occur when both grids hit the bottom of a 60hz cycle at similar instants - as you might guess, it's much less common there than at sites with more usual single power.
Also, don't forget that since this is a timing thing, even small perturbations make a difference. The D700 may be operating at a nominal 7 or 8 fps, but it may also have a slightly different release lag or AF recovery period, neither of which would be surprising. 60 cycles per second means 16.6ms between cycles, or 50ms return over a three-cycle period. With 37ms rating for the blackout period on the D3 and D3s, and 4ms for 1/250th exposure, it might just turn out that it's easier for a D3/D3s to "find" the bottom cycles than for a camera with, say, 44ms blackout period... etc.
_____ Brian... a bicoastal Nikonian and Team Member
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