Hi all,
Not sure if this helps, but when I had my R32 GTR, I broke a couple of speedo cables. When the speedo wasn't working steering started off light, then got heavy. Not sure if R33 uses speedo cables? But R32(atleast my 1990 model) does. Maybe your cable is getting worn and its having issues reading the speed?
Long shot I know, but worth a shot(atleast for SiR_RB)
Cheers, James