Looks like the answers are starting to reveal themselves. if when these items are replaced and if there is still an issue check out the water pump. If it has one of those plastic impellers they can fail gradually and limit the flow through cavitation or loss of drive. I think its the thermostat though, failing to have full range in opening and closing. If the O ring for the housing isn't in the pack and you want a quick try out, boil the removed seal in water... three minute egg style, if its undamaged and it should reform it to near original shape.
