Another possibility is that the gauge needle is sticking at 40 psi. This would explain why it sometimes shows a low reading of 40 when you are at freeway speed, and also sometimes shows a high reading of 40 when idling. Next time it shows 40 inappropriately, give the top of the dash a slap with your hand and see if the needle springs to the normal pressure reading. If it does, you've found your problem. If not, I would replace the gauge sending unit and see if that solves the problem. Good luck!