1969 Cadillac Deville backfiring - troubleshooting

69 cdv 472 90000miles

steady backfiring (lighty) in every 2-3 seconds when cold (like 'spitting back' in carb.)
starts fine - with no cranking when warm

when warm - idles well, no backfiring, not rough
when I gun it in 'park' or 'n', loud backfire occurs
if I slowly increase rpm during idling - no problem
when driving, it does backfire every time I accelerate - no matter how slowly I accelerate

I drive the car once a week for about 30 min.
The problem went away a few weeks go, now it started again.

steps taken ---
changed breaker points (bought cheapy set though.)
set dwell 30, timing approx. 5-8 BTDC
firing order is good (15634278)
cables appear ok - not too old.
dist cap and rotor not too old.
put 'water remover' in gas, sprayed a can of carb cleaner through carb

ignition coil appears rusty (it's an "OEM"shaped entry level Jacobs coil) - few years old
this coil - I guess - has some sort of a 'fake chroming', now like 75% of coil surface is covered with rust.

thanks for any advice