I had a gun one time that I used only a flat oil based enamel in it. It had a lot of solids in it like primer does. When the job ended I thoroughly cleaned it and used it for wood stains for several years. Then an employee broke my good gun and I needed one for shooting clear finishes. I took this gun apart and thoroughly cleaned it again and started using it for the clear and continued to use that gun to shoot clear lacquers for a couple of years. Then one day I was spraying the finish coat on a mahogany table top and it spit out specks of the oil based flat paint I had used 10 years prior. Ever since then I have guns that only get a clear finish put in them. Sprayers are just impossible to get completely clean and some finishes are just harder to get out than others. The best chance you have of getting the gun clean is to completely take it apart and put it in a container and soak it in lacquer thinner. Then use a small stiff paint brush to clean it. If your gun is spitting out specks of dried paint its a good indication the gun needs more cleaning.
As far as the gun spraying like powder, if there is any obstruction in the sprayer preventing the paint from drawing, it's like you used a nozzle too small for the finish you are using. What happens is it sprays more air than paint and the particles of paint just dry before they reach the surface you are spraying. You have to either fix the sprayer so it sprays more volume of paint or thin the paint more so it doesn't dry before it gets there. Some finishes like lacquers you can add retarder thinners to it to make it flow out and dry slower.
The paint you are using I would suspect the thinner for it is rated for different temperatures. If you use a thinner made for winter in summer it can cause the finish to spray like powder because it is formulated to dry faster.