Power lose at 7000 feet

Question: RV 4 equipped with Rotax 914 engine and veriable pitch propeller.

Problem observed during flight above 7000 feet , vertical speed become 0, rpm over shoot for 2,3 second up 6300 when turbo engaged for gaining height.i did effort for Turbo but not gaining height. Feels like under power. But take off was normal. Air Box target pressure normal.All other engine parameters also normal.

So come back to pavilion.

Suggest possible causes please.