I don't know how the oil pump on a Jeep works, but on SOME oil pumps, the impeller is driven by a key in the shaft (usu. a distributor shaft). So if the key breaks, or the keyway breaks out of the impeller, it will work just fine until it slips. Then it will not develop proper pressure, until (unless) the broken-off key snags its other half again, when it will work again. Sooner or later it will just fail outright.
I don't think I'd ignore this. It could get expensive.
BTW -- is the oil level OK ? That could cause a similar symptom.