260z oil gauge behaviour opposite to what was before

Previously my Z's original oil pressure gauge worked in the usual manner: at idle hardly no pressure shown, at higher revs the pressure would go up. I knew that it is the way these often work and wasn't too concerned. FYI I have Mallory Unilite dizzy and before it was installed with Mallory Coil with NO Ballast (this was done by the PO). Tach wasn't hooked up and wouldn't work even if I connected it.

Then the head was off for porting and new cam. Soon after my Mallory coil went bad and I got a new MSD Blaster 2 and Ballast. I installed them and got the tach working too. But next thing that happened was that the oil pressure gauge now works the opposite way: at idle it shows some pressure but when the revs go up, it indicates nothing at all, goes all the way down. Now, I'm not running that engine before I solve this and I am going to get a mechanical gauge to make sure I have pressure. But I would like to know what can cause the oil pressure gauge change the behaviour into completely opposite to what it was before? Is this common and any ideas what might be the reason? I don't know if it is because the tach is now connected but might be. It would be nice to have the original oil pressure gauge working so I'd like to solve this so that I have both tach and oil pressure. As a double security I will have the mechanical gauge too.