You can turn off the hud (press the mi/km knob in). The hud does connect to the speedo, so it shouldn't be a problem, but turning off the hud will allow you to check that.
1) Loose connection behind the harness2) Speedo module is on the way out - most likely - you can either get a working speedo from another car: runs about $100 if you can find it; or get a speedo module from
http://www.kaido-sports.com for $200, these are JDM brand new (not rebuilt) direct from Nissan to address digital speedo problem -- first check to make sure that your cluster doesn't have burnt out circuitry in the back - I believe there is a thread which shows you how to remove your cluster here or on zilvia.net.3) Speed signal generator (in engine block) is bad?