Is this a new build ? Or was fine then stared happening?

Have you started it cold with the cap off and watched the tstat open and water is flowing ?

Have you tried removing the stat and put just a restrict or in?

Heat gun the the rad top to bottom, and the hoses what’s it telling you .

Anything odd about performance? Pinging or sluggish?

If it just sat and idled would it boil?

Check the timing at idle , seem right?

It is making pressure right?