Can you tell us what your cold/hot idle oil pressure is? If it's not 40'ish cold or 25-30'ish hot, there's a chance that you've got a rotting/cracked o-ring at your oil pickup tube and you're sucking air. If your oil pressure is good, etc. then I'd look at replacing the VLOM. That's what sends high pressure oil to the AFM lifters to collapse them. The AFM lifters are durable and are sort of fail-safe (e.g. they use oil pressure to collapse them - not keep them extended), but they do not react well to a lack of oil or really dirty oil.
Anyone with an AFM motor such as the 5.3L LC9, etc. needs to be changing their oil every 3,000 miles without fail. Non AFM motors can handle longer intervals.
If you're pulling your heads off already, look into a bronze bushing trunnion upgrade as well. It's cheap insurance on high-mileage engines against loosing a needle bearing on your rocker arms. Any DIY person can do them.
Anyone with an AFM motor such as the 5.3L LC9, etc. needs to be changing their oil every 3,000 miles without fail. Non AFM motors can handle longer intervals.
If you're pulling your heads off already, look into a bronze bushing trunnion upgrade as well. It's cheap insurance on high-mileage engines against loosing a needle bearing on your rocker arms. Any DIY person can do them.