Increase the number of occlusion rays to adjust the quality of your ambient occlusion map. If the problem remains, create a custom map in the Transfer Map tool (Lighting/shading > Transfer Map) and connect to it the mib_fg_occlusion shader. Ensure that Trace is turned off for all target meshes; otherwise, they may occlude the source meshes. (Open the Attribute Editor for the object's transform node and expand the mental ray section. Under the Flags section, select No for Trace.) Adjust the Accuracy in Final Gathering section via the Indirect Lighting tab in the Render Settings: mental ray tabs.
The most important attribute for improving the detailing is the Max radius. A smaller Max radius value produces better details.