Idle was rough before harness replacement - trying to diagnose the rough idle led to the harness replacement when the wiring crumbled. Another data point - engine ran way smoother, almost perfect, when outside temp was below 35 degrees - the colder the better.

Mechanic replaced the plugs, checked the plug wires and said they were good. One knowledgeable type said that one or more of the coil packs might be bad - is there a way to test that theory without replacing them?