My ATXRasPi has just started doing the same thing. I built an AllStar node a month or so ago (Pi 3B, UHF radio, and ATXRasPi) and it's been working fine until recently. Powering on/off exactly as expected.
Now it behaves as described above - it powers on for a few seconds, starts to boot the Pi, then it shuts off the power. It does, very occasionally, complete a boot successfully.
I've tried the pull-up / pull-down resistors as suggested above, but in my case that didn't seem to help (assuming I interpreted the instructions correctly).
I don't think it's a lack of power, as the input is 40A 13.8V --> 5.25V 3A DC-DC buck --> ATXRasPi (measured as 5.26V input, and 5.22V output to the Pi). Voltage stays around 5V during the (very brief) boot sequence. I've used good quality wire for the DC connections too.
Any suggestions?