No oil pressure warning on steep inclines

[ToryGillett]

Good morning folks, so I recently purchased a used and most likely neglected 2006 Volvo Xc90 (176000 miles) and driving it home had a couple warnings light up the first was a low coolant warning which I have since taken care of.
The second warning and my current concern was a "no oil pressure" message this warning showed up as I was coming up the top of a steep incline. I pulled over immediately and checked the oil level and it was fine, after a out 3-5 minutes I started the car back up and the message was gone and the car drive for the most part just fine.
After getting it home and going through a bunch of routine maintenance items and such replacing filters, spark plugs oil, etc. The car is running pretty dang good. Last night my girlfriend took the car out to meet some friends. And on the way she encountered a fairly steep hill on the way to her destination and again the no oil pressure warning came up but quickly went out after leveling out.
What should I be looking at to remedy this situation? The car seems to be running very well other than this particular instance that has happened twice .