There are a number of reasons for this to happen:
1-The fast glossy interpolation settings are too low: to correct this, simply change the interpolation grid density to 1(same as rendering) or higher (i.e. 2 (double).
2- Bump and/or displacement values are ridiculously high: to correct this, simply decrease the values.
3-The physical light intensity values are ridiculously high.
4-An object with an artifact in the scene is being reflected.
One general approach to correct this “flickering” glossy artifact is to select the material/s of the object/s on material editor, and check/enable their "highlights+FG only" function, under the "reflection" group, of the main material parameters. Note that, this function will disable the reflections however, it will retain the glossiness.
Users often resort to this technique to avoid spending too much time searching for the rogue object/s in the scene.
Finally,more often than not it's attributed to one common mistake: the luminance of some glowed object/s in the scene are set to "unitless", under "self illumination (glow)parameters". To correct this, simply change it to "physical units: (cd/m2)" instead; and increase its default value,if necessary.
I hope you have found this post useful.
My 3D Portfolio:
New Book: 3D Photorealistic Rendering: Interiors & Exteriors with V-Ray and 3ds Max
More tips and Tricks:
Tips & tricks for architectural Visualisation: Part 1
Essential tips & tricks for VRay & mental ray
Creating Customised IES lights
Creating a velvet/suede material