That is normal, the top is always hotter because the hot water comes into the radiator and its cooled across the length of the tubes as air flows by the fins, provided the tubes aren't plugged up, a radiator always works like that. Drain the water and it will start out warm and get hotter as the water at the top finally gets to the drain.
Are you sure the thermostat is good? There have been odd cases where a brand new (dealer!) thermostat was defective right out of the box.
And is it mounted the right way? Some have discovered that they installed it up side down, in the later engines they made it impossibkle to do that but in the 617 its possible to install it the wrong way.
Try running without any thermostat (some will tell you not to do that on a engine at least on a long term basis) if its still steaming then your water pump must not have all its impeller blades (ask gsxr what that looks like!)

$1900 for a head gasket? too much! Its doubtful that would be the case anyhow, unless you are losing coolant (I mean not out the overflow pipe)

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!