I saw that processor sometime went into ISP if it worked in noise environment. What to do with RxD pin, cap will effect UART functioning. What will be with pull-up resistor? This is an experiment and I don't have watchdog running.

1) You can add a pullup resistor to the limit of your 232 converters drive capability, that will help if the noise is picked up on the board, but most likely it is not (see a) below)

2) make sure the 2 units (PC & uC) are grounded together

3) make sure the shield on the 232 cable is grounded in one end only

4) depending on your baudrate, you can add a small capacitor but I would advise against it.


Then
a) if this is a flywire board, that is probably why
b) you are not one of those that believe you can work reliably without a MAX232 or equivalent I hope.

Erik