Electrical issues
 

I really need from help from the more experienced on here. I have a1994 model GSX600F. been running very well for many years. This winter, garage got very damp. Tried starting the bike and there was an electrical flash from the positive battery terminal when pressing the starter button. Dried battery top but it happened twice more. Then started fine. Ran well until last week. After 20 miles, I parked for 15 mins and returned to the bike. Wouldn't start and no power to lights, horn, dash lights, ignition, nothing. Fiddled around with + battery lead and bingo, all lit up and fired. Ran normally 20 miles home.
Logic says its a break in the 29 year old + battery lead but however much I twist and fiddle with it, I still get continuity with a meter.
Can anyone advise what might be going on. Could the original flash have damaged something in the CDI or starter motor? PLEASE HELP. I'm a bit scared to trust it for a longer run in case it fails again.