I've been practising on the Cessna 172P. What I have found at time is that the vertical airspeed can oscillate widely at times, when I am attempting a level flight with a sensible throttle setting (say 2,200 RPM) at a couple of thousand feet. I know from experience real light aircraft don’t behave like that (currently learning to fly)! The oscillations are say +1,000 FPM to -1,000 FPM
One thing I have wondered if my HW is partially responsible for this. FG is running on a 3.2Ghz Pentium with 8GB of RAM, a SSD and NVidia GT1030. I have noticed that the CPU is running at 100%, but that could just be FG grabbing all the cycles! I’m loathed to spend a lot of money upgrading as it’s really just a ‘scratch’ machine. I’d rather change some of the settings to reduce the CPU load or make the aircraft more stable. My aim is really to get the plane more stable so I can practise navigation etc. Not to have the ultimate graphiical experience.