280z overheating only in the rain
 

So a purchased a mostly 280z back in the spring of this year and had only ever driven it in dry weather. Today I decided to take it out in the rain to see how it did. Let’s just say it didn’t go as good as I had expected. It was idling good when it got warmed up in my driveway, but as soon as I started driving it was acting weird. It was idling at 2000 rpm at stop signs and then started to overheat till I stuff it off. Whenever I drive it during the dry weather it idles at 900 rpm and stays right at 180 degrees. It does have a new 180 degree thermostat (that I put on when I bought it because the old over had said it sometimes runs a little hot) and a new radiator(this was put on by the old owner) . Does anyone have any answers