914 MAP issue

Good day everybody, I had recent trouble on 914 MAP value. I was on steady and leveled flight with my aircraft @ 170Km/h, 4700 rpm and 26 MAP approx,  when the MAP slowly increase autonomous up to 35.8 MAP and I felt power lost, and because of that, speed decrease consequently. 

This unbelievable MAP rose up to 35.6-35.8 had 40-60 seconds steady duration after that it come down to 26 MAP again, and power become normal again with speed increase.

This issue appeared during the same flight three times  with MAP value increase (different values even over 34-35), temporary power lost and disappeared within one minute approx.

During the entire flight (30 min approx) the 914 engine was running without any rough or trouble; it was running like a clock.

Engine is know under check now by authorized  shop but anyone of you had the same issue or meet this issue and found what has been defective ?

Roberto