Jakob, I'd hold off on purchasing a new harness.
Yesterday I was about to purchase a new ignition coil, it had quite bit of corrosion on the secondaries. A few minutes and a small amount of baking soda from the kitchen later, she's clean, tests out fine, and I'm back in business. That would have been $130 for no good reason. The spark plug leads are shot, they'll be replaced.
Do go through every single connector on your bike. Clean each terminal until it's bright and tight. Add a little dielectric grease on clean terminals and they should stay that way. Replace the glass inline fuse with a blade-type.
Temperature can most definitely affect conductivity through a system. Electricity warms things up, temperature makes things grow (at different rates), sometimes breaking connections. Especially dirty, corroded ones. Or crowded itty-bitty ones like on the ignition and kill switches 5twins suggests.
The harness is the backbone of the electrical system. But maybe your intermittency is in a switch or other component? A new harness wouldn't fix that.