Hello Zungara,
Had a similar problem with underextruding. The diameter of my filament was varying from 2.85 mm to 3.2 mm.
When you expect diameter changes you have to measure in more directions since sometimes you can have an oval shape.
Also when your extrusion is fully stopped and measure the diameter just before the feeder, the extrusion the problem might be already gone.
When my filament diameter becomes above 3.05 mm it does not fit through the owden tube anymore.
When the diameter rises above 3.20mm it is diffcult to get the filament out of the bowden tube.
It looks like a blockage in the hot end, but after taking it appart several times I never had a blockage with the V2 Hotend.
Instead of getting another filament supplier, I used a 4mm ID bowden tube instead of the supplied 1/8" ID tubing.