Replace the Thermostat
Almost all overheating problems, or excessive heat in cooling system ends up referring to a few basic parts. The first and cheapest is the thermostat. Just change it, and if the bolts are as bad as you say, file down the sides until the next smaller size socket will fit, then replace them with the thermostat. If you water pump is failing you will see antifreeze seeping from the weep hole on top of it's housing, that's why they drilled the hole there for, to tell you your pump seals are toast. The last item would be a head gasket leak which would show mostly white smoke during operation at idle, regardless of engine being warm or cold, more pronounced before operating temps, because coolant is trapped in the block until the thermostat opens.
My best guess is that the thermostat is not OEM and has failed. Order the thermostat off Fastlane before you start your dissassembly, so you can go ahead and change it out.