It sounds like your house electric system is grounded in more than one place (you said the previous owner was going directly to ground instead of running a neutral to complete a circuit).
It seems prudent to go to ground at every opportunity, and many folks drive ground rods all over the place thinking it's safer. In fact, the difference in electric potential between grounding locations can actually cause current flow between them, especially when one is actually being used to complete an electric circuit. This may be why you see a weak spark in your ground circuit.
Trying disconnecting the circuit with the offsite ground and see if you still see those weak sparks. If not, there's your trouble.
We found out the hard way that we only have one ground to our well. Lightning struck near our well a few years ago and fried a lot of stuff in our house, including our TV.