ESD problems with PIC 16F88

I have built a number of battery operated circuits using PIC16F88s.

They are housed in a plastic case with a socket to plug in a serial
connection for controlling LED outputs by RS232 comms.

Sometimes when you touch the socket with your finger, the PIC responds
in a manner suggesting that it has received some random data on the Rx
pin of the USART. The problem occurs much more frequently in cold dry
weather which I know is worse for static generally.

Presumably this suggests that electrostatic discharge is being
interpreted by the PIC as a signal.

Any ideas on how to prevent this?
(Ideally with modifications that I can do to existing circuits as I
have made quite a few already.)

Here is what I have already tried, in an attempt to fix it:

- Now there is a 1K resistor in series between the USART Rx pin and
the socket pin corresponding to serial data.
- I have also tried a 100K pull-up from the Rx pin on the PIC to Batt
+ve.
- I have attached 100k pull-down resistors to the other unused pins of
the PIC that were previously not connected. (They are all set as
outputs anyway)
- The PIC's Vdd has a 100nF capacitor connected directly between it
and Vss (within 20mm of pins)
- There is also a 10uF capacitor between Vdd and Vss (but located a
little further away on the board - about 150mm from PIC)

The PICs I am using are SOIC package. I made a very similar batch of
circuits a couple of years ago using DIP package (also PIC16F88)
running the same program and didn't have any such problems.

Thanks in advance for any advice,
Bo.

.

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