Try to keep it simple, process of elimination
Sounds as though you know your car pretty well and am convinced that the probelm corelates to the head work.
I would think of all the items the mechanic would have interfaced while performing the repair and which could have been altered.
sound like you've replaced most of the low hanging fruit already. I would think it's possible the fan, which I am assuming was removed in order to do the timing chain work may have had something happen to it and it is no longer functioning as intended.
Stick to the basics, sounds like it's cooling fine at speed and under load but heats up at stand still. I have found bad radiator to exhibit opposite tendancies, can idle all day but under load car heats up.
Sounds like could be an airflow at standstill as long as you've rulled out mixture, presume you've redone mixture yourself and you are satisfied with concentration.
Probabilities are high your change in running temp resulted from recent work.
There are only so many components and chances are it's got to be one of them. Coolant, radiator, airflow. water-pump, thermostat....