Hey onyx, the Path Follow operator by default uses regular forces to push particles along paths...even just slightly tweaking the trajectory of a particle along a certain axis can engage a sort of butterfly-effect, where further into its lifespan its trajectory will be way out of whack. That's not a bug, just moreso a property of chaotic systems.
One thing you could try, if you want particles to follow the 'disk' shape of your path exactly, even if it has bumps in it....use the spline you have now as the force to move your particles, and then create a cylinder in the scene to act as the surface the particles will stick to. Object Bind your particles to the cylinder (surface mode, and sticky set to 0%) and then add the bumps and things to the cylinder where you need them, rather than the spline. That way, the spline will keep your particles moving around but the cylinder will keep them in a flat disc.
One thing you could try, if you want particles to follow the 'disk' shape of your path exactly, even if it has bumps in it....use the spline you have now as the force to move your particles, and then create a cylinder in the scene to act as the surface the particles will stick to. Object Bind your particles to the cylinder (surface mode, and sticky set to 0%) and then add the bumps and things to the cylinder where you need them, rather than the spline. That way, the spline will keep your particles moving around but the cylinder will keep them in a flat disc.