Before I turn to head gaskets, want to rule out some things first. $138K miles, 2001 Diso II. Yesterday on way back from beach, started to overheat. Pulled over, notice it was belching coolant from the around the base of filler cap. Let it cool off for an hour, added fluid, continued on. I could drive for about 5 mins and temp would stay level, but no hot air coming from vents with heater on. Then after five minutes temp would slowly rise. I stopped, fluid spilling from filler cap and let it cool off. Proceeded forward again with same result at which point I called for a tow home. I did notice the top two hoses from thermostat were rocket hot, but buttom hose from radiator was literally cool to the touch. Today I replaced the thermostat (old one was only a year old). Just went on a test run and got the same result; level temp on gauge for five minutes, then it slowly starts to rise. Once stopped, fluid continues to leak from filler cap until cooled. Running to get a filler cap now. Any chance the water pump (only 1.5 old) is malfunctioning?