overheating? completely STUMPED Options

[mjoslin1990]

I've replaced the rad, the t-stat, and upper rad hose. I'm car is driveable for about 10-15 miles then it starts to get hot (I pull over immediately).

The water pump is working, and so is my fan. My coolant is actually going back into the overflow tank and NOT cycling back into the radiator (SOMETIMES when its completely cooled is goes back, but not always)

I understand the most common issue is something as simple as the cap. I actually REPLACED the cap and it STILL was overheating (I'm thinking its a bad cap because the FIRST t-stat I bought WAS defective).

Say when I get my new cap (I put my old back on the new one is even worse, doesn't seal, and its the right one) I still overheat.

What could possibly be the next thing to look at? Mind you water pump DOES WORK, NO LEAKS, and its NOT the heads (did a pressure test and others).

Ideas????
__________________
~Miann