i run the tyres on 40psi usually, to no avail.
the castor is fine.
im sure its something getting old and stiff, i keep thinking the knuckle, casue when i replaced it, it did heaps (to fix the heavy steering i had), and now its not so good.
Basically u have to turn the car, and then unturn it, like the steering wheel hardly spins back the other way unless u make it.... its that bad.