well imo there is one of 3 things
1. you have somehow crushed a bearing but its hanging on im thinking a main bearing the center one this could have occurred as with an oil cooler and remote filter it takes some time to build oil pressure in the lines and cooler if the motor has been sitting for a while it drains back down if this is so if you have started it and a bearing has picked up on the crank loosing clearance i have seen this before if the motor needs a rev to keep it running
2. the sensor you are using is faulty or there is an issue surrounding the sensor try using another oil pressure sensor and try when in neutral in the garage
3. the oil pressure relief vale spring has broken and at idle there isnt enough pressure to open it relieving the pressure to bypass but at low loads it is enough pressure to bypass oil but with more load comes enough pressure to bypass and lift the total oil pressure can you give me an idea on what sort of pressure you have at idle and when the problem occurs in more detail rpm range and psi pressure
cheers
mat