Since you say it only happens when it's rainy, I'm wondering if water is washing some grit in and stiffening the chain a bit. Next time it skips, stop and back pedal while holding some pressure off the RD idler cage so the lower loop is slightly slack. Look for any links that don't immediately open up straight as the chain leaves the lower pulley. If you see any, there's your problem, and the solution is to change your chain care method to something that works better in the rain.

Rain might also somehow affect your cables causing poor trim in the higher gears. After upshifting to smaller sprockets, try plucking the gear wire away from the down tube like a guitar string to ensure that the cable pulls all the way down. If the plucking works to prevent or reduce your problem, it's either in the cables, which need a lighter lube, or in the ergolever which is resisting the RD spring and not returning to the correct location.