somtimes when coasting or idling, oil temp drops from 70-75'C to to 55-60'C rapidly. as soon as I free rev the engine, it jumps to normal reading again. I'm wondering its just my gauge/sensor lags or there is not enough oil flow at that port at lower rpms ?

I have oil temp sensor screwed in the lower allen bolt port in this pic below
and the pressure gauge in the upper one

 

Hey from what you have said above I would put it down to possibly lack of voltage going to your temp sencer. And yea I'm not an auto sparky so anyone else feel free to correct me.

 

sounds legit..