Now that I'm a little more awake, and have had a chance to think about this a little more, you might have a leaking gasket somewhere allowing coolant to seep into the A/F mixture portion of the manifold when the motor heats up and the metal starts to expand a little. In my line of thinking, this could cause a lean condition and depending on the science involved and where it is actually leaking, the resulting steam could produce a little back pressure against your primarys and perhaps cause the feeling you have at the gas pedal.

Now, please note, that I am not an expert, and I have never seen anything like this happen. This is simply my hobby and I may be completely out to lunch here; but, I figured I'd throw it out as a possible theory and in turn learn from the real experts if could be possible.

Anyway, good luck.
Dutch