At this point Alan, ALL comments are welcome. Willing to try anything.
On a previous board, i cut the +5 + 0v tracks to tracks to the processor & replaced with shorth length 0.5mm dia wire with a series diode and a 100uF and 0.1 at the processor end. It reset 23 times over night.
My current thinking is that either the processor brownout is dodgy, or the MAX202 devices are noisy and from time to time, pulling spikes on the power supply, or lifting the earth rail. However I'm having trouble believing that 100uF at the processor end of a series schottky diode to the processor as suggested by MWS, would not filter out all spikes. But it didn't help. The board you see has just done 2500 iterations without failing. Time will tell :)
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