I have a 304 engine. When it starts up it will have 55 pounds of oil pressure, when it warms up it has about 20 to 25 pounds of pressure. Which I know is normal for this motor, but when u idle it up cold or hot the pressure will drop to zero. Sometimes it rises back to 10 to 20 and other times it dont. It has a new bottom end in the motor. Which is the crank and bearings. What might be causing this?