zd30 coolant gauge playing up causes?

Hi guys been a while, but i now have a 2001 GU Zd30, it has a new head, new Radiator, all the anti grenade mods, egt gauge and boost gauge. I have had it on the road for about 8 months no worries till todayout and about driving and all of a sudden the coolant temp gauge starts playing up. The gauge would drop to 0 when i accelerated and climb again when coasting.
Any ideas as to what it may be? sensor? bad earth?
Its not loosing coolant and the egt temps seem to be ok, boost is set fairly low (12psi i think from memory)