2000 5.3 oil pump issues?

[Cdshinne]

I have a 2000 silverado that has 187k that has started to develop some oil pressure issues. The other week it decided to drop all pressure and the lifters started ticking like crazy. Changed the oil and put lucas in it the next day and it ran 40-60psi and never dropped bellow 40. A week later after driving 40 min i mashed on the gas to pass a car and lost all oil pressure then it started ticking like crazy. I limped it a mile down the road to get home on 20psi and parked it. I started it up the next day and have been driving it for 4 days since and no issues.

I am thinking oil pump but I thought when they go out they are out for good, I dont think its the oil pressure sending unit or sensor because when it hits zero lifters start ticking. I dont get it, and any opinions would be appreciated!