The first thing I look at when the engine breaks down at higher rpms, is the coil. I have had more than 1 fail. First sign was roughness as the rpms come up. Sometimes can not run over 3000 rpm. Could also be an issue with the points and condensor if so equipped. Check the wiring between ignition box, coil, and distributer.
'75 CJ5 304cid, t18 tranny w/granny low, model 20 T Case, DANA 44 rear, 30 front, 4" suspension lift, 33" tires, in need of more work!
"The only thing necessary for the triumph of evil, is for good men to do nothing." (Edmund Burke)