I agree with the photobleaching diagnosis. Except for reducing the excitation light with ND filters, you could also try to get rid of oxygen in your solution, either by bubbling N2 before use, or by using vacuum for a while. Oxygen mediates the photobleaching, if you get rid of it, things work a lot better. You can also look into the bag of tricks from microscopy; antioxidant additives like ascorbic acid or propyl gallate may help. There are also several oxygen scavenging mixes available that continuously eat up O2 in your solution. Martijn