100W LED going dim/bright/dim/bright every second after 10-15 seconds of power
I'd suggest monitoring the current flow. You've not specified what the rating of the DC-DC converter is, but in order to get the full 100W out you'll need something like 10A going in to it. What you might be seeing is either the PSU or converter hitting a current limit, going into safety shutdown, and then turning back on again.
Note that if you actually want to run a 100W LED unit at 100W the light is extremely bright and the unit will get hot, so you need to think about mounting it to a heatsink.