912ULS occasional engine surging

I took the plane to Lockwood in Sebring, FL for a 5-year rubber replacement and general checkup of the Rotax 912ULS. They installed a new fuel pump, replaced carburetor floats, installed new spark plugs, checked hydraulic tappets, sync the carburetors, adjusted idle, etc. and i still had the problem on the hour flight back home!!!   Any ideas on what to check next?

I had a similar symptom which got worse over time. Finally tracked it down to an air leak past an o-ring on the stem of the fuel tank selector on/off  valve. At higher power settings, enough air was getting sucked in to "foam" the fuel supply to the carbs and mess up the a/f mixture and/or starve the engine. This *does not* show up as reduced fuel pressure because the diaphragm pumps move air as easily as fuel.

I also once found a problem similar to what Jeffry Stetson described, except that the bad o-ring was at the gascolator.  It was not leaking fuel, but would suck air and foam up the fuel.  We found it by temporarily installing a section of clear fuel line that let us see the bubbles. This was on a Piper with a Lycoming, but the effect was the same.  

Despite logging in/ out, I am still occasionally losing my response - just happened again. 

I will add my afterthought - don't entirely dismiss a fuel flow problem.

       Large flakes of contaminant in tank outlet eg paint, tank sealer, insects, etc can cause an intermittent flow reduction.

       Visual inspection of gauze filters may not reveal  a translucent contaminant eg cloth fibers 

Sam,

Both you and a neighbor suggested i might have arcing problems, so i moved the sparkplug wires slightly and I believe the missing improved, but it didn't go away. Next, I plan to get some help to check the sparkplug wires for visible arcing at night, but it could be arcing between the wires in the bundle. Can the wires be replaced in the old ignition coils and plug connectors?

I looked briefly for replacement sparkplug wire systems, and they seemed expensive, whereas the wire alone was more economical.