If your original problem was paint going on very dry, apart from thinners another factor could be that the air temp was too high?
I sprayed once on a 28 deg. day and the paint dried before it hit the job and gave a good sandpaper finish!
Also your spray pattern of 5cm X 2cm doesn't sound right, I would have thought a longer, thinner fan pattern would be better.
300psi also sounds very high.Try using the minimum pressure that still sprays a good pattern. Just keep dropping the pressure and testing till you reach the right point. Too high a pressure just puts paint into the air and less where you want it!
PS. Don't take anything I've said here as gospel, as it's just some points that I've learnt by trial and error