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no CEL or anything? do you still have the stockers that you could reinstall to see if the new COPs really are to blame?
CEL's rarely come on if its a COP problem.. and i cant put the stockers in bc i blew them all... i know for a fact its gotta do with the new COP's. it never happened with the stockers
its probably the ignition switch. I know i had the same problem one day with my 83 5.0, one of the little tumblers went bad in it and it would crank, but wouldnt run unless i held the key to a certain point. I would check the ignition cylinder/switch and see if that fixes the problem.
try taking a look at the cps (cam position sensor). may have gotten water in there. even though u don't have a cel there still might be some codes. if you don't have a tuner go to autozone and check it out.