okay seems the freezing issue is pointed to the relation between VTT and vDIMM.
Here's the test.
vDIMM 1.65v VTT 1.28v CL8-8-8-20 2000Mhz -> 32M PASS
vDIMM 1.62v vTT 1.28v CL8-8-8-20 2000Mhz -> 32M FREEZE
vDIMM 1.65v VTT 1.34v CL8-8-8-20 2000Mhz -> 32M FREEZE
vDIMM 1.65v VTT 1.28v CL9-9-9-20 2000Mhz -> 32M PASS
vDIMM 1.62v VTT 1.28v CL9-9-9-20 2000Mhz -> 32M FREEZE

I'll say from this test my chip doesn't like VTT to be too close to vDIMM...