LED brightness stability issues - how to fix?
Ditch the buckpuck and run the LED off of the constant current mode of the LAB supply. If it is more steady then it's probably the buckpuck. 

Another thing to do would be to check the voltage supplied to the buckpuck, some of these cheap lab supplies dont have a very steady output. 

If you really need stability an LDO is probably the way to go in constant current mode (except the load would be the LED, and you need to deal with thermal issues of the LDO):


High Side Constant Current Source

Here are some other cool ideas for stabilty:
www.ti.com/lit/ug/tidu922a/tidu922a.pdf