I could just sink that right now!
Know what you mean about yellow Jan. It IS the pigment that causes the problem, whatever shade or brand is used. Like Wojtek, I always try to do a primer/undercoat first, very often thinned matt yellow, then do the final coats in gloss, no matter what final finish is required. If possible, it's better to spray yellow, as brushing is what causes the problems mostly. I don't know the 'technical' answer, but basically the pigments and carrier are 'tumbled' and partially separated when brushed, drying as a streaky and or 'lumpy' finish. As you say, a pain in the whatsit!
strange thing is, on a 'full size' subject, if applied smoothly, yellow can give a fantastic finish. I coach-painted my second Escort rally car in 'Rescue' yellow, and boy did it shine! Never won anything though!!!