914 losing power at hi altitude

Have you investigated the plumbing associated with the 3-Way Valve on the Air-Box.

If the Valve transfers too early, you will get an overly rich mixture. (Partial Flooding)

If the Hoses have air intermittent Air leaks, you will get partial fuel starvation events.  <---<<<

Power loss at high altitude

I was flying around 10000 altitude at 40% throttle with 914. The problem I faced that the AFR ratio started to lean as indicated by the sensor. EGT 2 and EGT 4 started to drop at 400°C to 500°C. Due to this the aircraft couldn’t maintained its altitude.

What could be the possible reason behind this?