Isuzu Repair: No Heat, 1998 isuzu hombre, vacuum lines

Question
I have a 1998 Isuzu Hombre 4 cyl.  I have no heat and the temp gauge moves from 100 to 155 deg.  At 155 deg, it does heat but quickly stops heating when the temp gauge starts going down.  I have changed the thermostat and checked the temp. sending unit (it checked good).  While I was under the hood, I found a vacuum line that was bad and replaced it.  That seemed to help a little as far as keeping the heat going longer.  Do I need to search for other bad vacuum lines or is there something else I should be looking for.  Thanks!!

Answer
Absolutley Chuck. Well, if your system is overcooling, then you could have too cold of a thermostat. These models should heat up to about 180-195 degrees. Your heater core might be at fault. Or your blower motor could be at fault. Also check for vacuum lines and your temperature sending unit for any faults as well.