1999 Ford Focus power steering

Anyone able to offer advice on a rather irrating problem with my power steering.

The steering seems to be going through stages of being ok'ish & then heavy.
If I drive for a distance on motoways etc it seems to require less effort to turn after being on the motorway than before.
I've changed the P.S fluid (using the turkey baster method)I've also changed the P.S pump the pressure sensor in the pump & the supply hose to the pump & not a jot of difference.

Could it be the high pressure supply hose or the return hose getting blocked.

I've ruled out (maybe incorrectly) dampers when I removed the presuure switch it was fine for a day.
It's not leaking fluid & I don't see bubbles in the resevoir when someone turns the wheel lock to Lock.

Help please I'm stumped!

Thanks