For any catalyzed system you will probably want to have a jar of methylene chloride on hand to occasionally soak the metal parts of your gun. Note....methylene chloride is the NASTIEST stuff you'll come into contact with...so suit up properly when handling it. Methylene chloride is the basic chemical in paint strippers. You can buy Jasco at Home Depot, or instance. Pour a little Jasco in a GLASS jar and soak your metal parts overnight. Jasco will break down most plastics so don't soak plastic parts if you want them to be there in the morning.
Swish the metal parts around in another jar of acetone to remove the Jasco.As Brian said.....use a fairly cheap HVLP gun to shoot with. Clean the gun thoroughly after each session with acetone by using a half a cup of acetone and shaking the gun as you spray it out. Repeat this two more times and the inside of the gun will be pretty clean. The metal parts only need to be cleaned with Jasco rarely. The pin and the orifice are the critical parts on the gun. Keep them clean and a cheap gun can last quite a long time. I'm spraying with a ten year old gun today. I've put every kind of chemical through it....and I've used it a LOT. Of course...use a cheap gun and don't be afraid to throw it away and start with a fresh one.
Also...strain your finishing material as it goes into the gun. Often contaminates are the cause of the spraying problems you described.