69 camaro with built 307 (similar to 302) runs fine in cool temps and in higher temps on open road. However, at higher temps runs very rough. Have replaced pressurized gas cap and t-stat. Any ideas?

What'* the timing set at? You might want to try dropping it a couple of degrees just to see if if has an effect.

What temp t-stat are you running?

Are you running electric fans, or the stock pulley driven one? If pulley driven, is there a viscous clutch on the fan? If so, have you tried running with a solid fan mount?

Has the rad ever been replaced? Have you ever put bars leak or anything like that in it?