I know you mention brand new parts, but I just wanted to point out it's still possible you got a faulty part. Sucks, but possible. I know a few have complained about do88 quality issues lately, including
deansbimmer
It does seem like it is something restricting flow, either a bad/blocked rad, or a tstat not opening fully could cause this.
edit, he mentioned it here:
https://www.m3post.com/forums/showth...8#post31780043