PA-32 pilot reported engine malfunction during cruise. The flight diverted to an appropriate airport.

Narrative

We had some engine work done on the plane in the time prior to this trip. The exhaust system and one cylinder had been replaced. We had test flown the aircraft without incident prior to this trip.I flew from ZZZ1 to ZZZ2 without incident. On the return; I was on an IFR Flight Plan from ZZZ2 to ZZZ1. I had been cleared direct ZZZ1. When I was over ZZZ; I noticed that the engine was beginning to run roughly. At this point; I was able to confirm that there was no valid EGT reading: we had lost cylinder one.I elected to [request priority handling]. We flew in a manner generally consistent with ATC instructions and proceeded to land without event at ZZZ. I had the engine checked by an A & P at ZZZ before returning to ZZZ1. (The spark plugs were fouled with oil.) Cylinder 1 was lost ... again.... on the return to ZZZ1. Upon return to ZZZ1; we brought the plan back to the mechanic who cleaned and reinstalled the spark plugs. The engine failed run up coming out of the shop and we returned it to the mechanic who replaced the spark plugs. I then conducted a test flight by staying in the pattern at ZZZ1 and found that the engine was running smoothly and well throughout the flight. A runup the next morning confirmed the normal result.