Help!!!! Detroit oil pressure problems

Hi I hopefully somebody can help me as I'm pulling my hair out now. I have a Detroit diesel v12 71series (2stroke) that I am having oil pressure issues with. I know the pressures on there's engines are not mega high usually. The engine has been rebuilt and 'tested' by a local company a while back and just sat since and going by there sheet the pressures are all fine. All I done to the engine before it was fitted was fitting all the external parts needed. The book says 50-70 psi is normal and 28 minimum at 1800 rpm. Once it is started I only see 1 bar at 1800 and maybe 0.3 bar if I'm lucky at idle so it was shut down straight away. I have tried the obvious like a new gauge-no different. I have replaced the pressure regulator and relief valve in the sump- no different. I dropped the sump again and changed out the pump. Still exactly the same. The gauge shoots up to 5 bar on start up then drops and drops back to one bar. I'm completely puzzled. Any help would be much appreciated thanks