This could well be user error - I’m not great at soldering for example, but just wondered if you’d come across it before, as it happens with both that I’ve bought. Onoff shim, power attached to it, it attached to the corner of a pi3’s GPIO with the button on the outside corner, official raspi (5V, 2.1A) power supply, no other peripherals attached. The pi boots for about 3 seconds and then cycles. Boots fine without the shim.
Many thanks for any ideas.