The hop-up pulls the BB towards the top of the gun, no matter what angle. If you're shooting downhill the hop-up is going to pull the BB laterally [forward, relative to you], rather than vertically [relative to the ground]. This gives the illusion of the BB's over-hopping. Bottom line, when shooting downhill, aim low.
If you're shooting flat and it still over-hops, check your rubber for tears or wear. If it seems alright, wash it with warm water, clean your barrel and put it all back in and see if that helps.