C172 Flight Instructor reported while maneuvering they experienced a loss of engine power resulting in the return to the airport where they landed safely.

Narrative

We were conducting a cold weather steep spiral 1200-1300 rpm (and clearing engine) when we leveled off and began looking for a ground reference point. We were level for approximately 15-20 seconds looking for a point when we suddenly had a loss of engine power (not full loss but as if at idle). I added mixture to see if that was the reason; as well as turning on the fuel pump. Neither were affective. We then held level and some engine power was restored. We did not completely lose engine power (roughly 2000 rpm) but it felt like the plane might be headed towards failure. We held altitude and around 100 kts as we [requested priority handling] back to ZZZ. I chose to take the non active runway of XX to land. On the descent back to ZZZ our power was intermittent at half throttle (I was keep it at roughly maneuvering power). When I added or took out power the engine would start to die. We noticed any time we lowered the nose power would decline; so I carried excess airspeed into the landing in case we lost engine power on our descent to land. We did notice the cylinders 1 & 4 were in and out throughout the experience. Emergency vehicles did not notice any fluid or any abnormalities after we landed.