help - weird intermittent resets

Hi,

I'm getting a weird and frustrating problem with a pic18f252 that
intermittently resets itself.

I've reduced my test program to the bare essentials - pseudo-code is:

 - clear trisc.0
 - while true
     - reset portc.0  # turn on piezo buzzer
     - wait 1ms       # long enough to click buzzer
     - set portc.0    # turn off buzzer
     - wait 249ms

Config:
 - wdt and bor are on
 - 20 MHz crystal, with 18pF caps to ground

Observations:
 - pic resets at random times, varying between 1s and 20s
 - pic resets much less often when I disconnect the
   max232-based tty interface
 - pic resets less often with much smaller progs (eg, the above
   test prog written in c and compiled under ccs, as opposed to written
   in forth and bloated out to 4k by the vm and support code)
 - power supply is delivering rock-solid 4.85 volts
 - i cannot reproduce this resetting when running the prog via gpsim

Thoughts:
 - crystal is dodgy, locking up
 - pic is affected by ambient electrical noise

Any ideas on cause, and fix/workaround?

-- 
Cheers
David