tuning a magnum

I have a very similar combo and had a very similar problem. Mine is stock 5.9 mag bottom end. I have a custom ground comp cam with very similar specs to yours, stock heads with some touch up work, edelbrock air gap, Holley 750 DP, cheap headers. I did use a mechanical fuel pump though.

I had the same issue, it would pull hard out of the hole but once I hit about 4,000 it wanted to surge. Mine ended up being a bad fuel pump. New pump and it runs great. I would definitely guess it's fuel related in some way or another.