Raman spectra in Gaussian
Thank you for attaching your input file.
The first place to start for improving these calculations is your choice of basis set. 6-31G is a rather small basis set and is likely a source of significant error here. If you're fond of using the Pople type basis then you would be better off using something like 6-31++G* so that you include polarization and diffuse functions. With your current basis I suspect that you are missing important polarization of the atoms upon distortion.

If you really wanted a very accurate answer I would recommend the following scheme. Optimize your geometry at the DFT level and then use that geometry as a starting place for a optimization and frequency calculation at a higher level of theory. For example use the cc-pvtz basis set (a correlation consistent basis set that's more efficient generally than the Pople type) and CCSD(T). This will provide a very accurate gas phase raman spectrum. 

Edit:
The only additional Gaussian specific parameter you should need is the Geom=Checkpoint command for the CCSD(T) optimization and frequency run. This will use your existing checkpoint file as a geometry starting point. 
The other keywords (basis and method) are exactly what they should be cc-pvdz and ccsd(t). For more information on basis keywords I recommend the users manual.