no spark while cranking

1968 dodge dart .......slant six ....it does have electronic ignition........one day i noticed it was getting harder and harder to start .......soooo much cranking was needed ......i put an in-line spark tester in place and found that there is no spark while cranking but when the key goes into run position then it sparks .....so if luck would be there then it could crank a few times and and catch and start as the key would go to run ........once it starts it purrs like a kitten ........i changed the ignition switch (thanks TXdart) and no change ....i changed the ballast resistor .....no change .....i grounded the block with jumper cables while cranking .....no change ......any one have any idea what this might be .....this is never going to work on a cold morning LOL