What would cause speratic oil pressure to drop from 40 to 0 start knocking then back to 40 and stop knocking instantly on a 1999 GMC Sub 5.7L?

Since the problem is sporadic, it is probably in the oil pump or the screen or pipe on the oil pickup in the pan. If the screen or the pipe is loose, it can let in air which will cause loss of oil pressure. It could also be a stopped up screen in the oil pickup. GM lists the following as possible causes of engine oil pressure below spec: Oil pump worn or dirty Malfunctioning oil pump pressure relief valve Oil pump screen loose, plugged, or damaged Excessive bearing clearance Cracked, porous or restricted oil galleries Engine block oil gallery plugs missing or incorrectly installed Broken valve lifters Add to this, very low oil level. That's probably been checked, but in case it hasn't . . .