Posting this in case it helps someone with the same problem I had. I installed several sets of LED puck lights under my kitchen cabinets and they worked fine for about a month before they began to flicker and even fade. This happened independently (and not at the same time) to all 4 sets and had me puzzled for a while. These were sets purchased from Home Depot that had a switch (on the DC side of the transformer) and their own transformer which I had put into a junction box. I just left the switch on because I was using the wall switch to turn them on and off.

It turns out that the switches that come with these lights are awful. When I opened up the junction boxes I discovered that when I held the switch hard in the on position the lights would be brighter and not flicker. So I merely cut out the switches and spliced the 12V wires together. Problem solved and a lot less stuff in the junction boxes!