Mine worked great in my 85 w/493, wrecked it.
Put the digi 6 in my 87 while it still had the /6 in it, worked great.
Put the 500 in the 87 and like you, I had a missfire above 3,500 rpm or so.
Checked everything, sheilded everything and rewired everything, no help.
Noticed that when it would mis that the tach was jumpy. hmmm
One day out for a cruise I forgot to turn on the elec fan, flipped the switch and the tach started jumpin. hmmm
turned off all of the electrics and bam, no missfire.
Went to a 136amp denso alt and all is good.