Dave here are several guaranteed remedys the totality of them will insure spot free spraying.
1. Make sure the filter in the bottom of the gun is cleaned after each use.
2. Make sure the tip is the right size for the material being sprayed. Stains are finer tips than oil paint or latex 12 to 15 thousandths .012 to .015
3.Strain your paint with panty hose. I know this takes more time but we want it right the longer the gun works with out problems the more money you make faster.
4. Make sure the inlet strainer is clean too at the bottom of the syphon hose.
If you do all these things the gun will work fine.
One last thing if the spray pattern is fingering or not a consistant fan then the tip is worn out. Replace it. Apx 50 bucks.