My best guess is that your engine is over cammed and not intented for street use/crusing speed. Do you have a mechanical advance in your distributor? If so your weights and springs might not be right for those speeds, might even be broken, also do not use your vacuum advance if so equipped.
Outside of that, look at your power valve and adjust accordingly. Check your vacuum at idle if you have 8 inches of vaccum at idle put in a 6.5-7.0 power valve. Run 1-1.5 point smaller power valve than actual idle vacuum.