You know that the started doesn't have a ground strap(in this case it wouldn't hurt to install one). It uses the starter body as a return path to chassis ground. If you have high resistance between the starter body and the attachment point at the block you'll see arcing. There are two ways to do this. Begin at the starter and work backwards to the key switch or start at the key and work toward the starter. Check all the high current connections. BATT, solenoid, starter and even the heavy gauge wire running from the relay(solenoid) to the solenoid on the starter itself. Are all the terminals clean and tight? Are the crimps tight? You need an assistant to do voltage drop checks when it malfunctions. You have to check point to point at the time it fails.