Is it one particular mesh, or all meshes?
If it is one in particular, you may want to try exporting it into another format (LWO, 3DS, X, MOD, OBJ, MILKSHAPE ASCII), and then importing it. Sometimes a mesh gets so corrupted that unfortunately you have to start from scratch. I most often have this problem when manually creating faces off of an edge of the cylinder primitive.
If it is all meshes. Then try uninstalling, deleting the milkshape directory and then re-installing.
That is all I can think of, or at least fixes that I have attempted.