You described niter (or sugarsand) to a tee, 2 names for the same thing. You need better filtering. When packing the syrup, use a filter made for maple syrup, I like Orlon best, then use 3,4 or even 5 prefilters. Depending on the quantity you are doing, use cone filters if only batches of a gallon or 2, use flat filters if for much more. Flat filters need a rack, something to lay them in to then let the hot syrup flow thru. In either case as one prefilter gets plugged, lift it and very slowly dump it into the next pre-frilter. Repeat as necessary. In either case, both the pre-filters and the orlon filter can be washed, use hot clean water, no soap. Change the hot water as many times as it takes until the water looks clear after squeezing the filter in the hot water a few times. Just squeeze the filters, never wring them. If you wring a filter you break some fibers and essentially ruin that filter, it will no longer filter properly. Hang the filters to dry.
One thing also that can help is to add a little food grade diatomaceous earth (DE), available from maple dealers or if you just need a little, look into buying it from a local maple dealer. When adding the DE, stir about 1/2 cup in 1 gallon of hot syrup, then pour the mix in the filter. Keeping the syrup hot while it filters helps a lot.