Old Mega Works New One Doesn't!

I use a Mega 2560 to control a Preamp-DAC that I made about 5 years ago, everything was going along fine and then the system just shut down. I found that the Mega regulator had died. I purchase a brand-new genuine Arduino Mega (just like the old one) and installed it into the system, up loaded the code and everything worked fine, but for only about 10 seconds. The new Mega was still on but it switched the Preamp off. I turned the Preamp back on and it turned off immediately. If you disconnect the Mega from the power supply, then reconnect it, and then turn the Preamp on, you get the same issue as before, lasts for 10 seconds first time and turns off immediately every other time. So this is weird, exactly the same hardware (well newer I guess) same code, same install and it wont work. But wait it gets weirder, I replaced the regulator on the 5-year-old Mega and put it back in and everything works fine again!!!

Any thought on what might be causing the issue with the new Mega, I’m sure there are countless possible reasons but I was just hoping I was missing something obvious.

Any thoughts?