CAREFULLY take the sx6 and any soundcards out and see if it boots OK. Then put any soundcards back one by one, try again. Then put the sx6 back, try again. I cannot stress to be careful with manhandling the sx6 in its bare state.

Be very careful of the links on the underside of the sx6 board. If you suspect this may be temperature dependant (warming and cooling as the unit is used) maybe there is a soldered joint somewhere gone dry?

Personally I would use version 7a for the 6000 with sx6.