I would bet my money on the valve springs. I see you have installed a small performance cam. Did you install the recommended spring to go with the cam? Did you check the pressure on the spring you did use at installed height? New or not, you have to check the pressure. Even brand new performance valve springs can be too soft if not shimmed to teh required installed height.
If the springs are good, I would move on to the ignition system. Voltage at the coil needs to be a solid 12+ volts at all RPM.
What type of ignition are you using? If it is an HEI, there are many things that can cause the problem you describe, but most are terribly rare. Things like the module going bad, the magnetic pickup covered in oil and malfunctioning. etc.
I would start with the valvespring and work from there.
Good luck
RobT
