I have a Davis VP2 (with Aerocone for the summer, old cone & heater for the winter). I also have a NovaLynx 8" TBR and a CoCoRaHS gauge. When winds are light and rain is straight down, I have had close agreement between all three (+/- 0.01). When winds are significant they can vary by several hundredths of an inch.
I have calibrated the NovaLynx with their calibrator and checked the Davis against the calibrator as well.
I have a small back yard which violates the placement guidelines for rain gauges.
Having said this, I agree with others that the pre-Aerocone Davis seems to have a propensity for under counting, not over counting when wind is present.
It might be worthwhile inspecting the reed switch and connector in the ISS SIM. Also a frayed and intermittent cable could cause problems if the exposed rain switch wires are contacting each other.
Greg H.