Going down the electrical road,as mentioned above, the coil can break down under load if its faulty. As the revs increase the time taken for the coil to build up a charge decreases so if there is going to be a coil-related problem it will become worse as engine revs increase. I have found a simple High Voltage tester to be useful to see if a 40,000v coil will jump the preset gap on the tool. Plug wires can also be tested, one by one at cranking speed with this tool. Its handy if you can get a spring loaded push buton switch rigged to your starter solenoid so you can watch the spark as you crank it over.
( wire from bat+ to switch pole A. wire from switch pole B to starter solenoid +. )
If you have swapped out the coil for a "good" one maybe you could hook up a timing light and watch your advance curve on the HB to see if you have a smooth rise with the revs. Apart from plugs, coil, dist, leads and earths,and vacuum leaks, then you can only go down the mechanical road as co327 has suggested.
Last edited by 67Mustang Al.; 07-26-2012 at 06:07 AM.