Cadillac Repair: 96 Seville SLS Overheating, cadillac seville sls, cadillac seville

Question
I have a 1996 Cadillac Seville SLS with a 4.6L SFI DOHC V8 engine.  It keeps overheating on me and I cannot figure out why.  I've replaced the thermostat, radiator, overflow cap, and replaced a worn out belt that runs the water pump.  I haven't replaced the water pump or plugs yet as I have no idea if one of these is the problem or not.

It's leaking coolant through the overflow tank.  No hoses leak, and the system was pressure tested and came out fine.  Why would my baby be overheating like this?  Water pump?  If not that then what?  And how much would you estimate the cost to be?  

Thank you so much for any information you can give.

Answer
I would perform a back pressure test where you remove all the spark plugs and the thermostat and attach a compression tester hose to a constant air supply and screw the other end into each sspark plug hole and see if bubbles come out of the removed radiator cap. If they DO, Blown head gaskets. If they DON'T, I would go with water pump.