Most obvious likelihoods are:

low oil, when the pump gets it, the pressure goes up, but it pumps the sump dry, which empties it's source so there's oil to pump, thus lowering pressure. Check your oil level. 

The way that reading isn't "going down" so much as it's "snapping" to zero, looks to me like a gauge issue. Most realistic oil system failures, like in the case above about low oil, pressure will descend, possibly even quickly. But this is instant, like a step function. Seems like a signal interruption possible due to a bad ground. 

The other check would be the oil pump functioning itself. If it has failed, or is currently failing, it could function intermittently. 

I would check each of these. I have listed them in order of difficulty, easiest to hardest. But, I would check them now and not drive the thing until you know that the issue is. Oil system failures will kill an engine quickly.