Not to sound rude or confrontational here, but what you need to realize is that a head gasket doesn't just blow-out in a large section all at once... especially with newer vehicles (partly due to the primarily-metallic head gasket as opposed to the older graphite ones). What usually happens is that the gasket loses it's seal due to whatever conditions (stretched bolt, gasket flaw, whatever) in one small section and that section grows -usually pretty rapidly- to become the major problem that you're familiar with (the steam out the exhaust, bad/no idle, etc). Sometimes the gasket is just loose enough to allow coolant into the engine under a load (much like a tiny crack in a head or block would do). This tiny gap may stay that way for a pretty good amount of time until it finally manages to spread large enough to be very apparent (a larger leak). This is usually caused by an overheated condition that allows the metal near the site of the leak (which is already a hair loose) to bend (warp).
When the leak is this small and only under a load it will likely do nothing more than create a lean condition in the affected cylinder (which the ECU will try to correct by increasing the fuel in ALL cyliders to compensate -the ECU doesn't know that only one cylider is getting too lean). So basically it is not the easiest condition to diagnose when in an early stage like this -therefore any mechanic could easily miss it.
Obviously this problem may not be a head gasket or a crack in the head or block, but the signs are pointing that way. Really, I hate to see anyone have a blown head gasket and don't want to get on here and try to scare people with it, but "if the shoe looks the right size then it just might fit".
BTW if it was the water pump, the engine would overheat at idle or stuck in traffic.
Again, I didn't write this to be an a$$ toward you, mberndt. I just wanted it understood why I thought as I did