Reply to "Engine performance degrades quickly"

...Check Your #1 Cylinder TDC to the Zero 'Mark' on the Damper Balancer! There may have been slippage there causing an inaccurate Timing Setting.
Also sounds like a Streched or Warn Timing Chain, can be checked by observing with a Timing Light.

Could be a Vacumn Leak or Carburater Problems.

Why 'It' would run well, and then run poor; may be hard to find! Are You running a 'Points' Distributer? If so, the Dwell Setting may be Off, affecting Timing!

Last, check ALL the Plug Wires CAREFULLY for Grounding or Shorting Out Between Each Other, Especially at Higher Revs. Look for Arcing At The Distributer CAP, Near the Headers and Stainless Braided Hoses! A good way to check this, is at Night with All the Lights off, with a Second person revving the Engine while You Observe the Engine Bay. And Be Sure to keep the #5 and #6 wires completely Separate/Insulated to Prevent Cross-Fireing!

I would also check the Inside and Outside of the Distr. Cap and Rotor, for 'Carbon Tracking'!

A Lot of things can cause poor running. It could be as simple as the Wrong Spark Plugs or just the Wrong Gap!...

Good-Luck!