Alignment and resolution problem.

I've set up my grado gold cartridge as perfectly as I can on my mmf 2.1 using the turntablebasics.com alignment tool, set my VTA and VTF and azimuth as good as they can get. However, LP's tend to start out sounding good, but as the arm tracks inward toward the last two or three tracks, the sound gets grainy and sibilant, especially vocals. Is there something wrong with my cartridge that could be causing this problem or has anyone else even heard of this problem? Thanks guys,

Keith