what brand belt drive? some are prone to issues..
explain grind,,, as in when it grinds? some people call hard starting grinding.. just because the bottom end has been rebuilt,, doesnt mean its right,,, have you contacted the builder?
I have seen a good share of just rebuilt engines come through my shop that weren't done right.. nd had problems,,, shovel and evo bottom end rebuilds require expertise and a host of special tools and equipment...