PA-28 Flight Instructor reported engine RPM fluctations during takeoff roll resulted in rejected takeoff.

Narrative

We were performing traffic pattern operations on runway XXL. While taxiing back to runway XXL; we decreased power to reduce taxi speed. Upon doing this the RPM dropped from 1000 RPM to 560 RPM. In response to this; we increased the power to 1100RPM and restored power to the engine. We continued to the hold short line. At the hold short line; I pulled the power to idle to attempt to replicate the issue. I did this two times to verify and was unable to duplicate the issue. We requested a short delay to check for abnormalities. While holding the brakes on the centerline we applied full power. As the RPMs increased; I noticed the RPM very briefly fluctuated upon reaching 1100RPM but then continued to increase to 2200RPM. I released the brakes and had begun accelerating down the runway with all engine indication reading normal except the RPMs did not increase beyond 2290 RPM. I decided to take controls and reject takeoff as the RPMs did not increase to the usual range of 2380-2440RPM. We taxiied back to the ramp and squawked the aircraft for maintenance action.