During the motor rebuild which head gasket was used? The modern silicone impregnated head gaskets are reported to be problematic, and can blow as soon as within the 1st few hundred miles.
If any doubts, perform a Coolant Block Test, relatively inexpensive and very easy to do yourself, or take to a mechanic to have done. This test will tell you if you are getting exhaust into the coolant. The symptoms you are having could be this issue.
https://www.amazon.com/Block-Tester-...a-570352767565
Napa also has them, may have to be ordered.