Bushings may be "gone" in your dizzy, causing your dwell to vary wildly.........check with engine running and meter connected. Idle dwell may look okay, but increase RPM's to see if the meter starts to fluctuate. This is a common problem on old or highmileage engines.
If you don't have a dwell meter, borrow one..........either that, or pull the dizzy out, put it in a "soft-jaw" vise and check for lateral play and excessive run-out in the bushings.
Distributor can be rebuilt or just find one that isn't as worn.
Also, how worn down is the cam that opens the points? If it's worn down enough it will not run well above an idle. Lack of coil saturation.
These are only suggestions and are only worth what you paid for them
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