Running Hot on the Highway

The cap may be keeping the pressure too low, causing a vacuum effect and if the spring isn't in the lower hose, or its too short, it may be sucking the hose shut. I'm curious as to why the shop recommended this pressure rating on the cap, I always use a 16 lb cap. Just something to try, or ask the rad shop that re-cored the radiator. Also, its kind of a crazy question, but...is the fan belt tight enough?