As mentioned by a few, an air leak is probably at the root of the problem. Tight fitting of parts at the front rbd seems likely. If the packing initcat rear of barrel were at fault the filling of the pen would be inefficient and irratic or no fill would happen. In addition to items mentioned above by Ron, A cracked nib at joint with nib feed collar can do this, A cracked nib-feed collar that holds the nib and feed can cause this.
Sorry I haven't reported in a while, just been busy and all. I hope everyone is having a good holiday.
If there was an air leak as such, I believe it would have dumped ink at a much higher rate. This is more like burping effect than a leak effect. I haven't tried the pen since my last comment so I cannot update any further for now.
A quick question for those who have vac filled dorics: do they burp in general? Or are they capable of countering burps in their normal state (i.e. when fully restored and have no faults)?