Okay. It looks to me like V_Ang is not being affected by alignToVec at all -- notice how the green line moves as the vehicle climbs up mountains or tips into valleys. This seems to be the source of the problem -- the vehicle tilts up a mountain, and decides the target is behind it, so it goes back down the mountain, leveling out on flat ground, only to discover the target is behind it again.

This means alignToVec is either not modifying V_Ang, or it's modifying it incorrectly (I have just now noticed that V_Ang also modifies its two VECTOR* parameters, which is really a bad idea [my bad, that is -- that's how I wrote it] -- it should probably be modified to copy those parameters to two local VECTORs and then modify those, so that only the ANGLE is modified at the end).