One last thought.
Do you have an overflow tank?
It is the only way to insure that you have purged all the air from the system, and a recurring air pocket in the block could account for the initial spike.
I used an (repro) original style metal tank on the firewall, and properly modifies the cap.
One note, disregard the instructions to install it empty and empty it later. Always install it 1/3 to 1/2 full to allow the air to escape and coolant to be drawn back in.
