If the pen didn't have that problem before (which might indicate the wrong sack is installed), then it has to be the nib/feed/section combination.
Although the nib is sitting tight to the feed, it may be that they have different curvatures. If the nib has a tighter radius then the feed, there could be an excessively open space along the ink vein. i.g. The nib could be snug because it's sitting along only the edges of the feed.
Measure the radii if you can and compare them. A useful but delicate trick is to put chalk or pastel on the inside of the nib and fit it tightly to the feed by hand. That should leave tell-tail marks on the feed of the actual contact points, which should be pretty even.
Also, changing nib thicknesses can allow gaps at the edges of the nib. There, ink and air can move between the feed and the ID of the section causing blobs. The solution would be a different feed and a bit of "working" of the section.
Good luck getting the new nib sorted. I'm sure with some diligence you'll succeed.
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