Why could a Rotax 914 light aircraft lose power at 7000 feet? I was flying an RV 4 equipped with Rotax 914 engine and variable pitch propeller. 
The problem was observed during flight above 7000 feet; vertical speed became 0, RPM overshoot for 2, 3 seconds to 6300 when the turbo engaged for gaining height. I did make an effort for turbo but was not gaining height. It felt like under power, but take off was normal. Air Box target pressure was normal. All other engine parameters  also normal. So I came back to pavilion.

Could you please suggest possible causes?
Turbo wastegate probably stuck open.