No oil pressure in my 2000 !!!!!
 

Start the truck and pressure OK till 5 minutes later, pressure drops to 0???
could it be the pump or electrical failure...Please help, only truck :(

Does the engine make any kind of unusual noises when the pressure drops?
The best suggestion I can offer is to get hold of another gauge and tap into the same place your oil pressure sending unit is and confirm the actual pressure. Depending on the result will determine your next course of action.

yes, if I don't turn the engine off immediately and let it run for a bit, the valves start knocking and it just feels like no lubrication in engine at all.

From what you have just stated , I think we can eliminate any electrical malfunction.
The sudden loss of oil pressure can be caused by a few things. First, how long has the present oil been in the engine? It could be getting diluted by extra fuel getting into the oil supply. Yes, the oil pump could be on its way out. The pick-up tube could be blocked, the pressure regulating valve could be hung up in the open position. As you can see there are a few reasons for the low oil pressure and you will need to eliminate them one by one starting with the easiest first. Not trying to be funny ,but I hope the problem is with your car , a 2000 something ??? , and not your chopper !
I'm pretty sure you will get some more answers , hang in there. Last but not least I have seen where the oil filter was never changed and gave the same problem you describe.