My guess would be that it's the mercury on the move, as mercury can cause blurred vision and vision changes that can come and go.

candida can also cause this and DMSA can feed candida, so it's possible it's a combination. Watch your diet and make sure you're not consuming sugar, yeast or gluten grains (wheat, rye, barley, oats etc). All those things can aggravate mercury poisoning and with the chelation at the sametime, would make it even worse.