When was the last time the thermostat was changed? I had the same exact symptoms as you have and when I replaced the thermostat it cured them. If you haven't replaced it since you've had the car and you don't know when it was last replaced, then I would just replace that first. A thermostat is like 18 bucks and it's a breeze to install versus buying a water pump and getting it installed for 300. Try the small things first.