Amazing overheating problem...desperate

Interesting.... lift caps are known to be more iffy than non-lift caps.....maybe just a POC 16 lb cap. Brand?

And 13 lbs would allow it to boil more easily. Perhaps the lift cap and lower pressure and drilled is allowing the system to bleed the air out better. And may be the temps are colder.....

A small head crack will sometimes show up a a slight but persistent overtemp. It can be very mild; I raced one for several races that way and the crack was so small that it never really got super hot, just up to the H range.

Well, here is hoping you have it fixed!