Hi,
For anyone still interested in this, I had the same issue for the past four years on my 1994 R32 GT-R. The HICAS light would come on a few minutes after driving and the steering would go dead heavy.
I had Nissan and another workshop run the HICAS diagnostics multiple times - no problems: the back wheels did their flappy thing and everything.
Finally, just this week Custom Importz in Lonsdale SA found and fixed the problem. There were actually three faults:
1. The speed sensor in the back of the instrument cluster was faulty.
2. The steering angle sensor was faulty
3. (Not such an issue). One of the rear solenoids was faulty (this did show in up diagnostics but replacing it not fix the issue until the above two were fixed).
For anyone having this problem, follow the advice above to run the HICAS diagnostics, check fluid levels etc, but also get your shop to check these sensors in around the console and steering column. They were the "missing link" that did not show up in diagnostics.
Glenn