Erratic RPM and Clatter

Last fall I replaced my engine with another stock 5.0 because the oil pump driveshaft on the original engine twisted up like licorice. The replacement engine got new intake/exhaust and water pump gaskets as well as front/rear main seals. I put it in the car and along with some minor things like a small coolant leak and slight exhaust tick it's running kind of oddly.

The car will run and rev just fine but it will occasionally sputter. When it does sputter the tach goes crazy and bounces around. It usually will do this the most at around 3000rpm. It only does it sometimes though and I'm not sure what is going on. I also hear a bit of clatter, I can hear it best if I rev it to 3000rpm and then take my foot off the gas and let the rpms drop. Any ideas what it could be? I'm hoping its just bad gas since the car sat for ~9 months but I had Stabil in a full tank so I'm not sure...

No ideas what else might cause this behaviour? I went around with a propane bottle to try to find vacuum leaks with no luck

oldskolbus said:

How many miles on car/eng, what have you checked?

Have you ever changed the fuel filter or pump? Some pumps just die and others die slowley. The chatter sounds to me like a loose rocker.

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The replacement engine has about 190,000km/115,000mi and the rest of the car has about 140,000km/90,000mi. The engine is stock, out of an 88 5.0L. Everything is factory aside from the exhaust. I'm running the stock 10* timing at the moment.

The fuel pump is original while the filter was replaced about 20,000km ago. The pump is very loud though, and I did once have it die after a day of open lapping on a road course, but it fired back up when it cooled down.

Oh, and I pulled the codes to see if the computer caught anything, but it just gave me some random things that seem unrelated. EGR not responding, ECT sensor out of range, Park/Neutral circuit fault, and inoperative AIR system (I removed the A/C). You might be onto something with the fuel pump idea though.
I have only really checked wiring (loose grounds etc.), and for vacuum leaks so far. I suppose the rockers could be loose. I had the valve covers off the car but never checked them, assumed they were fine since its all stock/original.

Is it possible for an over-tight belt to cause an issue like this?