Can't find misfire

I hade this once ....stumped me for a looooong time... Take a good look at the distributor bushing in the block. The bushing can be seen after the distributor is pulled out, just look down the hole for the distributor and you will see it. If you have sloppiness in this bushing it can toss the timing all over the place. It wont generally do it under load because the bushing becomes "loaded up" with torque, but free wheeling it will do it as you explained. It may not be the issue but worth a check, cheap check, cheap fix if it works. I had one worn so bad it wore out the distributor cam drive...crazy! Hope it works!