Just a clarification-
Since you have several de-esser programs and your not getting the results you want I would use the above eq technique but use a notch on the eq. Make it a very small notch (by specifying a high Q value, which only effects a narrow range of frequencies, then boost the gain a lot) and move it up and down the freq spectrum until you find the correct freq. you want to remove...
Once you've homed in on the offending frequency range, keep the notch at that frequency but reduce the gain of the filter to a negative value to cut instead of boost that range and listen to hear if that works well enough.
That's the simplist and maybe least damaging option.
If you want to get more advanced, maybe because the above works and kills the sibilance but also does too much audible damage whenever the sibilance is not occurring, you can put that same high Q filter (boosted so the sibilance is super obvious and as isolated as possible) in front of the sidechain input of a compresser. Set the compresser threshold so it engages only when the sibilance occurs and you've just made your own de-esser the old school way - the compresser will only engage when the sibilance occurs, but it will squash everything when it does. Use the sidechain input of a multiband compressor instead, and only one band active which narrowly targets that frequency range and you're doing it the new school way - the compresser will only engage when the sibilance occurs, and when it does it will only affect that narrow frequency range.