I have an outdoor scene (exterior of a building) that I am trying to simulate the sun setting, going from day to night.  I am using Mental Ray, so I am prerendering the final gather map.  When I use a .fgm of the whole animation, objects rendered at night are very bright.  When rendering the same frame at night without using the .fgm, the scene looks good. 

From these results I am beginning to think that the .fgm file doesn’t apply to each frame, but instead to the animation as a whole.  This would account for the objects having way too much light on them at night in the animation.

Does anyone have any suggestions for how I can render this, without flickering, yet not have all objects overly bright?

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The GI and FG solutions apply to a single frame. For a sun animation, you cannot use the same fg solution for all frames. You have to re-make the fg map for each frame (or interpolate the results over several frames). Same with GI, as the sun will shoot the photons with different intensities and angles as the sun rises or sets.

For walkthrus, where everything is static except for the camera, yes, you can re-use the FG solution because the lighting is static.

For a scene where anything noticeable changes, you have to recalc the FG and GI. On the rendering dialog setup, indirect illumination tab, reuse menu, look at the “mode” options.