912ULS occasional engine surging

Could be ignition related.  You might try unplugging and re-seating the plug caps; or try all new spark plugs.  You might also try at a safe altitude over the airport, to swap mags in flight and see if the problem goes away with one particular ig box.  I've done that hundreds of times.  You lose a few rpms but it can help you troubleshoot.

I assume you have done the carb sync process.

I’m in agreement with Bert.  I find that typically an engine that misfires under full load, but does exhibit a noticeable misfire under part throttle, is more likely an ignition problem than carburetor related.  Start with inspecting and testing spark plugs, plug wires, and plug boots.   

Thank you Bert & Jeff. Tomorrow I will try the test of each ignition system over the airport.

I replaced all the sparkplugs and swapped the  mags in flight. But the engine performed as before. When I exceed about 5000 rpm it starts missing occasionally.

Check that the ignition wires aren’t arcing on the intake manifold and the trigger gaps are correct.