Three mechanical possibilities left:
1. Water Pump is bad and has scored the block not allowing enough water pump
flow.  (New German pumps from me $50)
The leak under the pump suggests a problem here as they are made to leak when
they go bad.

2. Radiator is bad and is not allowing the coolant to circulate properly.
New $195 from me.

3. Radiator Fan is not working (Do you hear it come on?)

The other thing that could be the problem is that you mechanic could be not
bleeding the system properly to get the air out.  Or could have possibly in
properly installed the head gaskets (don't see how but anything is possible).

Hope this helps,
Ken Wilford
http://www.vanagain.com
John 3:16
Office (856)-765-1583
Shop (856)-327-0027
Fax   (856)-327-2242