I take it you have removed unused objects and shaders from the scene via the edit menu?  Try doing a HD cleanup.  and see how many old temp files it gets rid of.  I have had the same problem with 7 before I went up to 8.  Usually these steps clear it up.  Oh yes, almost forgot, do a full reboot of the system after you clean up the temp files to make sure there isn't something stuck in memory.  It does happen.