The grit will be small pieces of food debris and possibly flakes of calcium from hard water deposits - both baked on hard during the drying process.
Rinse Aid is intended to let the rinse water drain off in sheets else it will form droplets and remain on the crockery and any crud in the drops will be baked on in the drying.
Try to rinse off the worst of the food before loading the dishwasher - above all use a good quality liquid rinse-aid rather than the multi-tablets - be sure to keep salt in the machine - clean the filters in the sump and consider a dishwasher cleaning agent now and again.
Inspect the spray arms for possible blockages.
Peccavi, August 2013
