250 Engine loses power and dies.

[JBM]

checking fuel starvation from a blocked breather in the cap is easy to troubleshoot.
attach a couple feet of fuel line to the petcock. hold it above the tank. open the petcock. blow hard into the fuel line. if you "pressurize" the tank,and it doesn't leak air out of the cap, then the breather is blocked. try running it with the cap loose, if the problem goes away, problem solved.
float height problems can also cause the same issue.
as can a hot/failing condenser. replacing and relocating the condenser up by the coil may help.