The oil pressure guage is a very common problem with the aussie R31, actually the whole instrument cluster can do weird things.
Good info here
http://darkhalf.r31skylineclub.com/guages/index.html
or r31skylineclub.com
Also.. i saw a R31 2 door coupe on jspec importers website last week. it had RB25 as well and looked sweet. Price was estimated between 7 - 11grand.