5th Gen rpm Buggin

[NightRider79]

whats poppin people? I'm new up on here. Well i have a huge problem, maybe someone can help. I just resently hooked up my ac and noticed that everytime i turn it, while it's raining my rpm needle goes crazy jumping up and down between the zero and the one. The engine doesn't sound like its die'n or anything, everything sounds normal. This only happens when i'm at a light or parked. Does anyone know why? i spoke to some mechanic but he said its the whole cluster thats malfunctioning, but the car is almost new with only 62,000 miles on it. Maybe one of you guys can point me in the right direction.