There were two topics about C172 autopilot, in either case, it was not clear what happened at first…
I never use autopilot… in the first topic, it turned out that AP was activated without my knowledge… and I met the issue, at first without knowing what was going on (or to get out !)
The second topic was posted by someone who does like to use AP, but ran into similar issues (instability).
In both cases, it turned out that keys used for aeleron and elevator are active during autopilot. MS/Asobo could disable these keyboard couplings during AP, problem solved. For self helpers: check your keyboard couplings and avoid aeleron/elevator keys during autopilot !