What I'm going to do is rip out the HICAS computer (I'm assuming it's the shiny silver box attached to the underside of the parcel shelf?) and check to see if there's any loose connections in it or the cable going to it. Because it's only every now and then, it might be due to a loose connection, so here's hoping.
Lozza, my problem isn't so much when it sits there idle, it's just randomly (around 30% of the time) when I turn the car on, HICAS light will be on and steering sluggish, and it'll stay that way until I turn it off and wander off, say to the shops. Even then, sometimes it'll be on again when I come back, but all in all, say 30% of the time. But yeah, checking the computer time, methinks.