You don't say what model you have but most wiring harnesses fail near the heat of the engine, for example as they go under the spark plug cover on a 104 motor. This causes misfires, rough idle or perhaps even inability to start. You will often get eratic and inexplicable fault codes. Sometimes touching the harness somewhere will cause it to run better or worse. But if you see crumbling insulation you're ready for a new one regardless of whether it's causing performance problems - unless you want to risk getting stranded.