imho change the C344C (to LM324 or proper equiv.) and then verify that the
cord to the handle, and all related contacts are ok. This includes in the
handle (there are two resistors in the tip - ~20 ohms heater and ~200 ohms
sensor afair - there must be NO leakage between them). The opamp dies when
the sensor resistor accidentally touches the heater circuit or tip ground
(loose wires in handle or connector etc). Happened to me several times.
Just replacing the 324 fixed the problem every time so far.

hope this helps,

Peter

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.