mmm...that's strange, and
Tamiya paints are normally quite forgiving. What thinner do you use? You probably already know but to be sure there are two good thinners in my honest opinion:
Tamiya Lacquer Thinner and Mr. Levelling thinner. The latter gives superior results (I usually thin 60-40 or more).
Tamiya Lacquer Thinner I use to clean the
airbrush with these days. Do you have a second nozzle and needle (my H&S Evolution 2 in 1 came with two different sizes of needle and nozzle)? If so, try out the other set and see if you have the same result or not. You might have tried it already, I'm not sure but I'm trying to think of a solution. Hope it can help, because a 4 months old
airbrush shouldn't give you overspray like this
.
Cheers