[Bruce Edwards]

Could be a few things. A lean mix, try pulling on the choke and see if it goes away, that is if you have a manual choke if not does it miss when it is cold? if not it is lean.
To much spark advance will cause a condition like you describe. If you have a vacuum advance you may have a bad one that is causing the spark to bounce. Try disconnecting and plugging the hose. You can also back off the timing a bit to see if it helps.
You said you went up on the power valve? The valve should be a bit under half of your idle vacuum reading. Normal valve is 6.5. Idle vacuum is normally 14-18. So half of 14 would be 7 to be safe a step down gives you 6.5. So if your car has a reading of 12 you would want to go to a 5.5 or less. The idea is to richen up the mix when engine vacuum drops below the power valve setting as can be seen at a high cruise speed.
That may also be why you do not see it in 5th gear as the engine is turning slower so the valve is not needed.
The accelerator pump have nothing to do with a cruise miss. What does the tach show when the car starts to miss??