I might try switching the printcore.
What I don't get is.This model is fairly large (13cm x 13cm) how can it be it's able to print the outside walls fine, while the inside gets underextruded? It's almost the same distance, acceleration and speed the printer has to go through on the in- and outside walls.
And yes I could try adjusting the different speeds, but shouldn't the printer be able to print With the stock ultimaker settings in cura.
I didn't mention this in the beginning, but it's not a problem that happens all the time, it may happen every 10-15 print.