Carburetor/power problems 912s2

Hello -first post here!

I have recently purchased a Tecnam p2002 with a Rotax 912s2 engine. Engine has reached 2000 hrs and been extended twice - we are currently around 2300hrs. Recently I've had an issue with the aircraft no developing full power and running a bit rough / vibrations. 

First time I noticed this was after a 50 hr check, run up was normal but just after rotation, power dropped from 2200 (FP) down to around 1900 - landed back on runway. Did another power check including full power stationary and a high speed abort and couldn't replicate. Tried another take off and same thing happened. Mechanic changed the spark plugs and checked the engine and all seemed ok. 2 days / 5 hours flight later, we had a reduction of power to around 2100 when in flight and some vibrations, this was improved instantly when carb heat selected and even after 5 mins of carb heat, vibrations returned if we put it cold.

Back at the field and another run up attempt resulted in 1700-1800 max with significant mag drop. Fuel is good, compressions are good and mechanic has found no sign of split /loose hoses etc. 

Any ideas where to look next?

Thank you.