Threre are some test we can make to narrow the cause of the resets.
First, let's try with clean backup ram. To do so, ooen the SD card in a PC, and in the BUP folder, delete any file starting with Fatfury3. Then indertnig back into the neosd, but before turning your mvs on, place.all the 8 dipswitches of the mvs board in the On position. Then turn it on. It should boot to a screen saying work ram test. Turn it off, and restore the dips to the original position (usually all off) and try fatfury3 again.
If it doesn't work, go to neosd ootions menu, and enable arcade mode, flash fatfury3 and turn your board off. Remove the sd card from the neosd and turn it on again and check if it doesn't reset.