[LMP]

As a first reaction, I think you have found the solution yourself when you said "I had recently replaced the coolant".
Unless proper procedure is used to vent ALL air from system, emptying and replenishing often leaves air trapped in the system and this has consequences very similar to what you experience. THere is a vent on the thermostat housing (probably) that NEEDS to be opened to let all air escape. ALso, there is a so called "vacuum" fill procedure.
Check for this possibility first before going for other causes. It seems the design of recent cars is more prone to this condition....I wonder why:it is like car makers continuously forget about past flawed design and keep reinventing the wheel.

If you have other signs...foamy white stuff on bottom of oil cap, milky oil, this would point to blown head gasket.....3.4 prone to that..