HOT UBEC (Buck adapter) replacement creates startup problem

The UBEC (12v-5v BUCK converter) was getting really hot, so I replaced it. Same symptom with the replacement. So, I ordered a UBEC with the “same” specs, but with a big heat sink on it. Now, it seems like the Arduino boots faster than the RPi and I don’t get any Blinky green light on the RPi - Unless - I unplug the power, wait for the RPi red light to go out, and quickly re-apply power. All good after this sequence.

My hypothesis is that the comms between the RPi and the Arduino/RAMPS are negatively impacted by the speed at which the devices now boot, primarily by the delay caused by the heat sinked UBEC.

So, here’s the question - Is there a way to slow the boot of the Arduino/RAMPS to help the RPi “outrun” the Arduino?

Second question - Am I approaching this all wrong and there’s a simple software/firmware “delay” I could introduce to make it all work? I guess I could just live with the heat, but this is Texas - heat=bad.

thanks much - just got my X axis working yesterday - hoping to light up the Y axis this weekend (have ordered the silent drivers for the RAMPS!)