I have a similar issue with my car and I was told to check the TFI module (Thick Film Ignition), which is located on the distributor.
My car breaks up from about 3k RPM on during WOT. The car also breaks up when I really bog it down going very slow in 5th gear. I have also noticed that is has gotten worse as I have modded the car, and it is worse when the weather is warmer. I also tried changing the plugs and i was told to try going a couple degrees colder with the plugs (25 down to 23). This seems to have helped a little, but the issue is still evident. I'm not sure if it is exactly the same issue you are having, but maybe it is worth looking at the TFI module.
I ran into a problem like that. After changing wires, plugs, cap & rotor as part of just a tune up and that didn't fix it. Next I replaced the coil because it was 20 years old lol. My data logs showed that i was running lean at high RPM and injector duty cycle was 95%. I replaced my fuel pump. as it turned out, the inlet for the pump was 3/4 clogged with trash. it would idle and cruise fine but wide open throttle just didnt have enough volume. might be some thing for you to check.