What kind of frame rates do you have in cruise? Low frame rates cause the simulation to slow down which might make it more unstable.
What kind of turbulence and wind do you have set up?
What weight was set for the aircraft?
I did a test in the A321 with no winds, at 34,000 ft, with 15,000 kg of payload and 8,820 kg of fuel. The speed started to fluctuate slowly. I was getting stable 60 FPS. This bug is very noticeable in the A321; it might be more subtle in the A320 and A319, but it’s something you should look into. This speed fluctuation is quite annoying.