I say as someone else did above, pull the thermostate first.

see if it is a flow issue. I think we have seen 2 or 3 like that in the last few months where the thermostate was bad, right out of the box.

It only takes a few mins to do and see if the flow characteristics change. if it doesn't help, then just one more thing to check off as checked.

also I doubt it is the lower hose, as you were at idle.