For the exact formulation you'd need a lot of maths and simulation, but if you only look for a feasible explanation, what Mike says is the easiest way to explain a changing gravity.
Big orbiting planets can stay at equilibria even when their orbits go one across the other, even when planets are almost nex to each other for a period of time. Therefore, you could explain the changes on gravity that way: When another orbiting planet goes nearby, gravity is way stronger towards that planet. Also, a big "star" (it's a planet, not a star, but people might be unable to differentiate) appears on the sky for that duration of changed gravity, which is really interesting from a mythological aspect.
If you want even more variation, add more planets to the "equation". That way, the mythology around that goes even deeper. Every planet, with it's distinguishable bright and color (since star's light are reflected differently) brings a different gravity and for a different duration of time.