Maybe replacing the coolant lines let air in, which is causing your Fast Idle Thermo Valve to act up. Not sure if that would actually cause it to rev to 3k on start up though:shrug: Follow the directions in your manual (owners or service) for coolant replacement, to get any air out.

As for replacing O2 sensors, that's one of the few things on a car that I'd recommend using a little anti-seize lube on. Some will recommend it on the packaging or even have it included.