[JT118]

The raingauge is a metric, there is a sticker '0.2mm' inside.
TX config is ok to 0.200.
Cumulus multiplier is set to 1.
It's perhaps a hardware problem, bad connexion or poor welding i will check tha tomorrow.
can the 0.1µF capacitor be problematic ?

OK, all worth checking, but it just might be an over reading Oregon. You won't know until it is all calibrated as above. Assuming capacitor and soldering is OK, It might be worth checking calibration. The Davis is a better instrument than the Oregon.