Tweeking the Auto intermittent wiper timing circuit ?

I've got a 1998 1.8 Xsara with a rain sensor to control the timing on the intermittent windscreen wipe.
Trouble is, the timing is sometimes a bit wonky and varies in an annoying
way (depending on the type of rain) . For example, sometimes it doesn't wipe the screen often as it should so that the screen fills with rain and then suddenly wipes at double speed for a couple of sweeps.
Does anyone know if it's possible to tweek the existing control circuit, or by-pass it with an electionic mod to give a more consistent timing of the wiperes ?
-- Steve B
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