Low-voltage LED Light string flickers when turned on, flickers go away after 2 mins
Most likely, it's the converter

Those are electronic power supplies, and they are made in China.   Fortunately they're also commodity items.  If you want a more quality version, try looking for a "meanwell" brand.  

It might be the wiring, and that is a serious fire-starter problem at any voltage, but that's not likely if powering off/on fixes it.