a rocket i'm making is unstable and idk what the cause is - anyone got a fix?

so i'm making a random rocket that's sort of like a "twist on the Saturn V", however the problem is that it's very unstable and when i lock heading, only swaying sideways at first before it turns uncontrollable
however, the problem is that i don't know if the instability comes from the rocket's design (position of fins and other structural components, as well as CoM/CoL/CoT positions) or from the PID's - i've tried multiple setups but all of them resulted in the same oscillation, however if i leave heading unlocked it remains fairly stable, so idk what causes the instability

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the rocket is intended for a lunar mission in VBSS (but any other real size solar system replica like HyperPatch's RSS should work as well) similar to the Apollo missions but with the whole thing being used as a combined lander and return module (rather than a lander and command module left in orbit), it's for a random "alternate history" fictional universe

if you do have a solution, preferably please only tell me what to fix, no need to upload anything