2006 Toyota Corolla RPM needle fluctuating a lot

I bought the car 6 months ago and even when I test drove the car, the RPM needle was fluctuating rapidly going from 0 to 4000 within a second, and suddenly dropping to 0 and so on. Even if I was at a red light, the same fluctuation happened. The dealer sold it to me saying that this was happening because of the sub-zero temperatures outside, but in the summer now, I am still facing the same problem.

I am not well versed in car problems and their solutions. Any pointers would be very helpful.