With the amount of research i have done there is a problem that Tamron are aware off but cant or are reluctant to fix,one guy sent his back twice with no fix and then the third time it was with Tamron for a few weeks,he contacted them and was told they where waiting for a new circuit board from Japan,this board never turned up so he took his lens back unfixed.
If the Sigma 50-500 was a little sharper at 500mm its the lens i would have,it handles better for me and to have 50-500mm in one lens is great.
I also read posts about people sending it in for repair, getting strange answers but never anything fixed.
I don't get enough freezes for it to bother me that much although one freeze at the "right" moment can get me all worked up for a while. A bit like when noticing my eye-piece dropped, for the zillionth time.