C182 pilot reported RPM fluctuations with significant vibration during cruise flight resulted in expeditious return to departure airport.

Narrative

Flew several loads of jumpers in the morning and they were uneventful. After my fuel stop I was flying a load of Jumpers up to 13000 feet ASL and when I got above 10000 I noticed my climb rate was slower than normal and my carb temperature was dropping. I applied carb heat to keep the temperatures above freezing eventually becoming no factor later on. As I continued climbing I noticed the RPM started fluctuating a little and shortly after the engine started surging first by over speeding and then dropping way off with a nasty vibration and did this a couple of times. Once I realized things were going wrong with the engine I made the call to drop the jumpers immediately to get them out if anything else happens. The first thing I did when I noticed the surges was pull the carb heat on and enrichen the mixture but this had no effect so I reduced power which reduced the overspeed events. I returned to the airfield and attempted adding power to see how the engine would respond while I was slowly descending over the field and it vibrated significantly. The aircraft was inspected and ran on the ground and the problems could not be recreated even during test flight. The oil pressure also was at zero around the time the prop started surging.