Mark - thankyou for your comments. ( the watchdog code was where I had been trying it on and off. All test results with WD off).
I tried several 0.1 monolithics, with very SHORT leads directly across the IO of the regulator, the supply pins of the uP and the supply pins of the RS232 transceiver, without success. Then put an 0.1 directly on the uP reset pin ( even though there was already a 0.1 cap on the reset line). No improvement - the board would do a reset about every 2-5 minutes, and R0 was set to 0. ( thanks Laborratte for that.)
Then i put a 1000pf on the rx pin of the uart and so far the board has been running continuously, receiving continuous data for 2 hours without a reset. I am guessing this is the solution. 1000pf is probably a bit brutal but seems to work.
I have had a look at the rx data with a bitscope computer analyser/scope and the waveform is pretty clean, is almost a perfect sq wave & runs between 0v and +5, and has about 10mv of hi freq hash on the hi & lo logic levels. This is a bit hard to analyse because there is some inherent hash in the bitscope display itself, (with the leads shorted to ground).
All in all, I think a good result.
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