Thanks @gordongreig !!! It really worked out.
I did change during the flight. My FPS dropped while in the air (around FL190) from 80 avg to 37 avg. I made the change and after a while it started to going up. did not reach the same amount, but just a few FPS drops
I restarted the same flight (MHTG to SEQM) without leaving the flight simulator (first) and then restarting the sim, and as mentioned the reduction was a few FPS only (around 2 in some cases)
As you said is quite unclear why, but it an workaround