Possible water pump cavitation?
It still sound like low coolant problem because of the erratic and rapid temp fluctuations. Possible air pockets in the cooling system.
You have a new block, but what about cracked heads? The small amount of white foam may be ok, but with significant loss of coolant (1 Gal) that seems to be problem that needs to be addressed a little more closely.
Do you have access to an infrared emission tester to put in the radiator? How about a block tester kit to see if the test fluid changes color.