I'm going to attack a similar problem this weekend and have been reading this thread. One thing I plan to do is to shut off the supply to the hot water heater to see if I can isolate the circuit where the leak is --- hot or cold. I'm not sure what I'll do with that information yet but it may...
Some "old" furnaces from 1998 were rated as high as 90% efficiency. Are you sure you didn't just replace the old one with a new lower efficiency one? Also, the numbers are close so there may be other variables including set point(temperature) errors, estimated gas readings vs. actual and others...
Sawzall is the tradename for Milwaukee Tools and it is the best --- comes in that red box! I've owned mine for 15 years and it has never failed me. I use it for everything including trimming my trees...
Apparently people don't remember the days when people living near power lines wound copper wire on a wooden barrel and buried it in the ground under the wires to get free power. Eventually the power company found out where the "leak" was.... :)
Personally, I would screw a jumper wire between the two and solder the screws for a solid, permanent contact. The arcing, if not stopped, could be a fire hazard if something burnable got in there.
It's hard to see exactly where that spark is but I suspect it is jumping from one piece of metal trim to another. Voltage can be induced into metal objects from high power lines. The first thing I would do is to ground the metal at an end to a ground rod and solder a jumper across where it is...
I never had any luck with solvents -- just melts the top of the glue and spreads it around.
What are you trying to do? Do you just need a flat surface for another covering or are you trying to stay with bare, flat, even color concrete slab?
If it's outdoors, I would fire up my pressure...