Nissan Patrol: Cold hose temperatures while running?

I have just had the engine in my Nissan Patrol ZD30 fully rebuilt. When I start the engine from cold it builds up temp as it should, but half a kilometre down the road the temp gauge drops off to zero. The top hose is reading about 45-50 degrees, bottom hose around 27. I cannot work out why it drops off to zero on the gauge, or why the bottom hose is so cool. I have checked the thermostat even though it was new and genuine, I have replaced both temp senders, have checked the water pump, and all looks to be as good as new. Any ideas?