He's already tried an oil pressure sending unit.
One thing you might try, but it would be a bit difficult I think to reach properly, is to remove the oil pressure sending unit, and blow some compressed air back through the port to make sure it's not clogged somehow, but I doubt that's the case.
ONLY other thing I can think of that would cause a gradual loss of pressure would be if you had a chunk of gasket material or chunks of RTV from a poor repair job that fell down into the oil pan, and are getting sucked up against the oil pump screen. If no one's been into any part of the engine then that's probably not the problem.
I'd still lean towards large bearing tolerances around the cam and crank but even at that it would have to be really really excessive for it to be dropping to zero on the dash guage.
You really really should try a mechanical guage hooked up to the sending unit's port. Since Im sure you have an old sending unit laying around you could use that to match up the thread pitch and diameter and come up with a guage and adapter of some sort to thread into the hole and check the pressure with a good analog guage to be sure. Hell it could even be something wrong with the cluster or PCM?? You'll never know unless you check the actual pressure reading though.
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2000 Dodge Ram 1500 Single Cab, 5.9 Auto, 4.10's, Auburn LSD, Edelbrock headers, 2.5" true duals with crossover, MSD Superconductors, APS Plenum Kit, Level10 Trans kit, Trans-Go shift kit, 2-4 BellTech Drop, SS/T rims, Toyo Proxes ST 285-60-17's, Sir Michaels Roll Pan and tailgate handle relocator, Keystone bumper cover with light openings, PIAA fogs, painted grille shell, painted Street Scene Speed Grille Inserts, SCT XCAL-2, Alpine head unit, Infinity Components front, Infinity Coaxials back, Infinity 500W 4 channel amp, Infinity Basslink, new dash, steering wheel, and guage bezel.
1996 Dodge Ram 1500 Club Cab, 5.9 Auto, 4.10's, Detroit Locker, Race Truck Trends Nascar Bumper, Painted SS/T Stripes, Alpine HU, Alpine Sound Field Processor, MTX Thunderform. 5.9L bored .030 over, Sealed Power coated pistons, balanced and polished crank, Comp 210 Cam, MP Timing Chain tensioner, Melling HV oil pump, 1.92 oversized valves and mild porting, Pacesetter Longtube Headers, FMS 24lb Injectors, Level10 Transmission kit, Trans-Go shift kit, 5 gear OD Planetary. Way more to come.