Obvious response would be the rack.  Steering pump can also, but rarely,
fail.

Simple response;  Check the air in the front tires.  Low pressure increases
steering effort.  After tires warm up they add pressure decreasing effort.

OUS

Just the other day the power steering seemed about 10-20% stiffer than
normal. Later it seemed normal again. The stiffness seems to come and
go. Still easy to drive, nothing violent at all.  Belt is fine, fluid
level just right, no leaks.  Has a 20 pence 'O' ring gone in the
steering rack and going to cost me ££££££'s.

Chris.