Slant won't drive over 25mph without cutting out

Arcing generally indicates an open or damaged coil/ plug/ other high tension. Might even be a damaged rotor. Anything that inhibits the path of the coil secondary voltage will create a "lightning" like condition, where spark is looking for a path to ground

Pull the wires, inspect, and check for continuity, including the coil wire. "Old school" use to be max of 1000 ohms per foot of wire. I've never seen a good wire anywhere near that high. Also pull the cap and the rotor and inspect both, and if it seems to have been dragging, or damaged, check the dist for shaft wear or out of true

It is not uncommon, with grease, cooked on debri, or a "swollen" cap that the cap fits overly tight and may have been cocked a little bit