First I apologize for the length of this post. I'll make it brief as
possible.

I bought a 1999 Dodge Ram 1500, 5.2L 4WD with 99k miles and drove it
for about 2 years without problems. I changed the oil regularly with
Valvoline 10W30 oil.

I parked the truck for 1 year without starting it. After this time I
checked the fluids, started the truck and let it reach operating
temperature. All was normal. Later, after driving 10 minutes (6-7miles)
the oil pressure dropped to zero. I shut the engine down immediately
and checked the oil. I started the truck after several minutes noting
normal oil pressure and drove the truck home.

I changed the oil and filter: Same problem.

I pulled and cleaned the pan, installed a new Melling high volume pump,
verified the pump shaft, and cleaned the pickup so that it looked new,
inside and out with oven cleaner: Same problem.

I installed a mechanical oil pressure gauge to verify the readings.

I pulled the pan and checked the pickup height. The pickup was tight,
and the pump was tight with a gasket. I pulled the valve covers. The
smooth black crust on the valvetrain and head concerned me. I cleaned
the head thinking the oil was being held under the valve covers and
running the pump dry. I had good oil flow to the rockers/pushrods when
I closed everything up. Same problem.

I pulled the pan to check if the pickup had ingested anything and
applied teflon tape to the pickup threads as a precaution. I pulled the
intake thinking the black sludge was keeping the oil there. No
problems. With the intake removed I was able to drive the pump with a
primer shaft. Oil flow seems to be what I would expect. Oil flow seems
to be uniform from the head in the lifter valley and draining properly.

The pressure starts at 70psi(cold) and drops slowly to 60psi(warm) with
the high volume pump (60psi & 45psi stock pump). If I drive the truck
the pressure drops slowly to zero and stays there. At a 3000rpm+ engine
speed, no load, the oil pressure drops rapidly to zero then stay there.
I may be hearing the oil pump cavitate or chatter at zero pressure. I
assume it is running out of oil but as near as I can tell there is
always oil in the crankcase, however it's difficult to accurately
check a dipstick with the engine running.

I've tried running 20W50 oil with no change. Crankshaft bearings
don't make sense in this case, however I'll check the endplay
tonight as I saw on another post. If crankshaft bearings were at fault,
why would the pump cavitate/chatter? I've put an incredible amount of
time into this problem already. I wanna' drive my truck again!

Any suggestions would be appreciated