I bet air pockets. Did'nt overheat in the past?
Take a look at the IAP tech section and under fiat look at the cooling system section and there is a bit on cooling system bleeding. Basically, make sure the front is high or as other members have done include a hose bib in the heater line.
80 FI spider
72 work in progress
2017 Golf R ( APR Stg. 1)
2018 F350 crew long box