I think the Isaac that you are researching is very likely Isaac Duffer of Amelia, Cumberland Cty VA. He was born abt 1733 and married Mary Vaughn. You can find him in the IGI files and the Ancesteral files at: http://www.familysearch.org/
I am researching him as well and so far I have not been able to confirm any parents for him. I have found a John Duffer of henrico cty VA brn abt. 1695 listed in the pedigree resource files at the above site. It seems likely that they are related.
I hope this was helpfull, please let me know if you are able to find anything more on Isaac.