sudden drop in rpm's

It has been a horrible last 3 weeks.  I was taking my highlander with a rotax 912uls on vacation.  Got half way and stopped to top off fuel.  Their pump was broke so took off to the next airport.  Upon takeoff about 500 ft, the engine suddenly went from 5200rpms to 4800rpms.  I returned to the airport.  Did everything I knew to check.  Attempted again and same thing.  Called everyone I knew including Lockwood in FL.  Tried everything that I could do there.  No luck.  Had friends come and help.  Still no luck.  Drove the 4 hours home leaving my plane.  Everyone said that it had to be something in the carbs.  580 total hours with a carb rebuild done just last year.  So, i bit the BIG expensive bullet and bought two new carbs.  Drove the 4 hours back with friends and installed the new carbs.  Seemed to work on test flights!  Well everyone left and the next morning I attempted to fly home.  Same issue!  So decided to limp home.  Made it.  Have tried to drain the tanks with fresh gas, checked plugs, changed out mags, new carbs, no vapor lock, checked prop, checked exhaust, etc.  When it happens the fuel pressure is the same.  5+pounds.  The egt's drop a bit due to the drop in rpms.  Battery good.  Oh, and on the way home my volt meter went from 12+ to 0 in an instance.  Maybe not related.  I have had to walk away for a few days due to the frustration level.  I have thrown a lot of money at it with no results.  Everyone I speak with does not have a clue on how to help.  I am reaching out here in desperation.