Hi, I wonder if anyone has seen symptoms like this before, or has any ideas what may be going on:

During the pre-flight post-startup checks, my Rotax 912 (80HP) performs as expected with a very slight drop in revs and slight change in engine sound.

However I have noticed during my checks, where the engine has been shutdown for a short time (e.g. 10 minutes), that the rev drop is significant 300+ RPM and the change in engine noise more apparent. This does not seem to be an issue if the engine has been shutdown for longer period (e.g. 45 minutes+), also post-flight pre-shutdown dead mag checks are absolutely fine - same as the regular pre-flight checks. I've tried the check at 3000, 3500 and 4000 RPM when the engine has been displaying the symptoms and the symptoms are evident at all of those power settings. There doesn't seem to be any issues achieving full power on take off roll.

I've tried changing the plugs and checked the resistance of the plug caps (all within +/- 1K ohm of 5K ohms).