If the valves are sticking it's because the machine shop didn't allow
enough clearance between the stems and guides. When the engine warms up
what clearance there is disappears and the stems seize in the guides. I
suppose the MMO makes things a bit more slippery.
The valves are sure to be galled by now, so the head needs to come
back off so the valve job can be redone. The spec on the clearance
gives a range of values. It's possible to be on the low side of the
spec and have the exhaust valves seize up, especially on the center
cylinders, which run hotter than the end one. Been there, done that.
BTW, if a machine shop doesn't install bronze guides it's usually
because they don't have the special tools for the job.