I have a somewhat recent rebuilt 400 that is just now starting to worry me with varrying oil pressures. When idling, it will drop down to no pressure. I know it will drop some, but it doesn't even register on the pressure guage. The oil level is correct, that was the first thing I checked. It comes up to normal when I accelerate. The engine doesn't sound or run any different than usual. I doubt it is the guage since it still varies from high to low and stays fairly regular when driving down the road at a constant speed.

What do you guys think I should do, check the sensor or just swap the oil pump? I'm wanting to swap the oil pump just to be on the safe side.

 

I would start by installing a mechanical guage to verify the problem. (I run the stock and a mechanical and notice very little correlation between the two.)

If you do have to go back into the engine for the oil pump, check clearance on your rod bearings.

 

Its a 351M/400 Block. These blocks were characterized by many things, one of which being an oil pressure issue. These engines are tough hogs and a drop in OP every once in a while shouldnt be alarming if no noise is present, just check the oil often and I agree with installing a mechanincal guage