I would say the EGR passage is getting clogged or the EGR valve is clogged or shot.
If it were a Chevy V8 I would tell you to pull off the throttle body & the EGR valve. Then clean out the port going into the manifold from the EGR valve.
I would pull the EGR valve & try to clean off the build up as best I can and get as much build up out as possible. You may want to have a shop vac right by the port so the build up comes out instead of going into the manifold.
Don
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