Mitsubishi Repair: 91 Eagle Talon overheat, full radiator, radiator fans

Question
Start with full radiator, first 10 to 20 miles, temp is perfect.  Then temp starts edging up then takes off and engine overheats.  As long as there is fluid in the radiator, temp is fine.  Something is pushing the fluid out through the overflow and then of course it gets hot.  It always takes a gallon to top it off, then it is good for another 10 to 20 miles, then repeat performance. New pump, new thermostat, new cap.  Same problem. Speed makes no difference.  Something is over pressurizing the system, the cap vents and I lose fluid.  Any Ideas ?  I write this as I sit by the side of the road.

Answer
Steve,
Overheating can be caused by many things.  You've replaced several of the common components which can cause the problem.  Other things that can cause your engine to overheat are a clogged radiator (or air being blocked from passing over the fins), bad engine temperature sensors which would prevent the radiator fans from turning on, bad fans, air bubbles in the coolant, or warped head gasket.  A warped gasket can alter the appropriate pressure flow of the coolant around the engine block to properly cool it.  Another area to check is the engine oil.  Engine oil not only lubricates the engine, but also help cool it as well.  Lack of oil or improper oil flow can also contribute to an overheating engine.  Have those areas checked out if you haven't already.
Good luck!