A King Air 200 Pilot suffered multiple inexplicable right engine failures despite appropriate maintenance attention. Maintenance ultimately determined the failures were due to foreign material blocking the transfer of fuel from the right outboard wing tank into the right nacelle tank from which the engine feeds.

Narrative

The first thirty minutes of flight was uneventful. Then at FL210 the right engine fuel flow fluctuated and the right engine torque rolled back and at that point the right engine failed. I secured the right engine using the checklist for engine failure in flight. Then declared and emergency as a precaution and asked for a lower altitude.I then proceeded to the nearest suitable airport and advised the passengers of our situation and that we were diverting. We proceeded to the diversion airport and landed uneventfully. The passengers and baggage were off loaded and I wrote up the right engine failure in our discrepancy book. I called my company and they sent out a Mechanic to inspect the right engine After completing an inspection there was nothing found to cause the engine failure. The discrepancy was cleared by the mechanic who scheduled a run up then a test flight. I got back into the airplane and started the right engine uneventfully then taxied to a run up area and did an extensive run up at takeoff power uneventfully. The mechanic reinspected the right engine and next I did a test flight once around the traffic pattern which was uneventful. So now the test flight was complete and that discrepancy write up was cleared and the Aircraft was returned for flight. I departed for our base and the first thirty minutes or so of flight was uneventful. Nearing the XXXXX intersection the right engine had the same problem as earlier and failed again in the same way as before. I secured the engine again and advised ATC that I had an engine failure and would need priority and direct to ZZZ. I maintained 11000 MSL and attempted a restart of the Right Engine and it ran for a while then failed again. I repeated the procedures and checklist as listed above and continued single engine and landed uneventfully. I wrote up the engine failure again and reported it to my company and Mechanics. It was later determined that there was some kind of label for a fuel cell or bladder that somehow ended up in the right outboard wing tank and worked its way to the flapper valve blocking fuel from flowing into the right nacelle tank at a rate to sustain the right engine so it failed both times approximately 30 minutes or so into the flight. I now know why the engine failed in flight and why the cause was initially unable to be determined.