If the engine runs a while and then starts to sag after a few minutes, it's likely going lean for some reason. It's overheating and then sagging as the compression starts to go. A bad plug won't usually cause this. You should check for air leaks in the fuel system, dirt in the carb passages, a cracked clunk line, vibration-induced fuel foaming, and so on.

A very common problem is a cracked clunk line. The flex tubing gets a crack where it attaches to the hard tubing, and starts to suck air after the fuel level drops to that point.

As always, if you put the engine on a test stand and it operates "all day" as long as you keep feeding it fuel, then it's not the engine, but something in the engine/airframe/fuel system combination.