Posted: 31/07/2014 at 11:34
Sounds like grape shanking. One cause is overcropping and with over 100 bunches on one vine that sounds like your main problem. That is an awful lot of fruit for one vine to ripen and I would reduce it by 75%. It can be very hard to bring yourself to remove promising fruit but it really is for the best. Start by removing all affected bunches then remove all small bunches. Reduce any overcrowding where bunches are interfering with each other.
Thinning individual bunches improves the quality of fruit and helps prevent disease so is also worth doing at the right time of year.