Misfiring...progressive loss of power again

[poweredbyford]

After about 500 miles of use the state of tune of my 302 powered Aurora slabside falls off in a steep decline to the point were it runs and idles so rough that it's hard to keep her alive.

I've had the car for about a year and the first time I replaced the carb, plugs,
wires,cap and rotor, coil in addition to an entire new fuel system from the tank foward and all was well.

Then the slow progressive decline began again. A new cap and rotor from MSD with brass contacts the last time and all was fine again.

Yesterday after an extended ride and about three weeks of trouble free running I thought I felt a drop off of power but thought that maybe it was just me but today when I fired her up it was more of the same...low rough idle and poor performance. Took it around the block and at a steady constant amount of throttle the power would come on and off almost like a switch being turned on and off.

Fuel pressure is good, plugs were good, no smoke of any color or any increase in exhaust fumes. I can't believe that I'm having so much trouble with stone simple equipment. Any thoughts as to why just replacing a cap and rotor makes a huge change and the progressive loss of tune afterwards.

Thanks,

Dale

PS I did have to replace the almost new Holley electric fuel pump which burnt out after about 1000 miles of use which seems to be unrelated to the loss of tune.