A BE19 pilot reported engine surging and fluctuating RPMs and a return to departure airport.

Narrative

While ferrying an aircraft from ZZZ to ZZZ1 for a panel upgrade; the engine began surging from max rpm down to about 1000 rpm and back. I advised ATC and turned back to ZZZ. I was able to restore normal power and land at ZZZ without issue.After pre-flighting the plane inside a heated hangar; the plane was moved out to the ramp. The plane started normally and was allowed 5-10 minutes to warm up. During that time a runup was completed and everything appeared to be operating normally. Taxi and takeoff were uneventful and I began my turnout from runway XX direct toward ZZZ1.At cruise altitude of 3500 ft; I began to reduce power to cruise settings. RPM was reduced from 2500 to about 2300. I then began to bring the mixture back for cruise. I moved the lever back maybe 1/8th to 1/4' from full rich position and the engine began surging.I immediately returned the mixture to full rich which did not solve the issue. I turned the fuel pump on and returned the power setting to max power. This also did not immediately solve the issue.I advised ATC and started turning back toward ZZZ. I continued to make adjustments to the power and mixture levers until the engine stayed on at full power; full rich setting. I left the settings alone at that point and began a slow climb back toward the airport; thinking that as soon as I move the power lever I would likely lose power again.The engine continued to run normally at max power and once landing was assured I reduced power to idle and landed normally. After landing; the engine continued to function normally.