Dave,
This is a real problem with the so called 'improved formulation', I would suggest that you could look at even one of those really cheap utilty sprayguns if you just want to blast on a wide coverage of a vehicle.
With Humbrol paints now it seems that no two tins are alike, you can take two from the same rack and one will come out well and the other tin will do just what you have described.
I looked forward to seeing what their new Acrylic range was like, but having picked up a colour card a few days ago from my local shop, I see that the range is 85 colours and they are only in the normal range like now. I had hoped for some mixes like their old Authentic range, but it seems that this is not too be. They are It was suggested by the manager, possibly to produce extra paints in the Railway range of colours.
Who knows you might get hooked and decide to look at a slightly more expensive airbrush and add a compressor and add to your skill range.
Regards,
John