Everything was proceeding normally, and nothing gave me any indication of anything amiss on that cold fall morning. After departure, I was given a climb toward Greenland to 10,000 feet. With Keflavik now behind me to the southeast, I leveled off at my assigned altitude and broke out the cruise checklist. After setting the power, I began to lean the fuel mixture to the same flow rate I had on the previous two legs from Belgium to Iceland. Just before leaning to the expected fuel flow, about 2 gph early, the engine gave a loud backfire. Not a good sound to hear when over rough seas beyond power-off glide distance of land. I enriched the fuel flow and started to carefully look at all the engine gauges. Nothing seemed to be wrong with the EGT and CHT readings, but two gauges were acting differently than on the previous flights. The oil-pressure and fuel-flow gauges were fluctuating abnormally. Had I not been keeping careful records on the first two flights, I might not have noticed anything wrong and would possibly have continued on to Greenland. Without hesitation, I initiated a turn back toward land and advised Reykjavik of the problem.