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