1996 Lumina intermittent problems

My 1996 Lumina 3.1 runs like a brand new vehicle most of the time. But sometimes I start it up and it sounds like a trashing machine. Sounds like the valves are clacking. When this happens it will not accelerate over 40 or 50 mph. Theirs a strange hollow sound at the engine something like a manifold exhaust leak .This may last a day or two and then it's back to running great. The engine is so quiet I can't hear it run.
Can someone give me some clue to what’s going on?

Generally, intermittent valve train noise can be an indication of lubricant flow problems (nt necessarily pressure problems. Poor flow can be caused by years of using incorrect oil or neglecting oil changes. Varnish and sludge can restrict lifter gallery and lifter bleed hole passages, prevent lifter check valves from sealing, and create valve noise. Short of tearing off the intake to inspect/clean lifters and rockers, you might try to drain the oil and fill with a good synthetic for a few changes to help clean up any accumulated varnish and sludge.

Generally, intermittent valve train noise can be an indication of lubricant flow problems (nt necessarily pressure problems. Poor flow can be caused by years of using incorrect oil or neglecting oil changes. Varnish and sludge can restrict lifter gallery and lifter bleed hole passages, prevent lifter check valves from sealing, and create valve noise. Short of tearing off the intake to inspect/clean lifters and rockers, you might try to drain the oil and fill with a good synthetic for a few changes to help clean up any accumulated varnish and sludge.
Good suggestions, Blue, seen abused engine like ( not this one necessarily) block the oil pick up screen, maybe he should go easy on anything that would loosen any sludge, could pull the pan and take a look.

What's all involved in dropping the oil pan to inspect the screen?

What's all involved in dropping the oil pan to inspect the screen?
Go to autozone.com/repairinfo for guidelines.

Generally, intermittent valve train noise can be an indication of lubricant flow problems (nt necessarily pressure problems. Poor flow can be caused by years of using incorrect oil or neglecting oil changes. Varnish and sludge can restrict lifter gallery and lifter bleed hole passages, prevent lifter check valves from sealing, and create valve noise. Short of tearing off the intake to inspect/clean lifters and rockers, you might try to drain the oil and fill with a good synthetic for a few changes to help clean up any accumulated varnish and sludge.
If it turns out to be a defective oil pump,what would be a reasonable price to have it replaced at the popular neighborhood mechanic shop.
P.S. This 1996 lumina has always had tender loving manitenance from day one,Always on or before scheldue.

Before you get excited about an oil pump, test the oil pressure cold and hot. With restrictions in the lube system, pressure at the rear main gallery will often be higher than normal.

What oil and especially what filter are you using?