Hi Chuck,
Firstly, is your gun a siphon feed or gravity feed model? In any event this problem revolves around unwanted air leaking into the gun from unwanted places.
In regards to a pulsing or fluttering gun -
Look to a Loose fluid tip.
Fluid tip not seated correctly in gun head.
Gun (with cup) tipped at excessive angle.
Obstructed fluid passage or hose.
Loose or cracked fluid tube in cup or tank.
Insufficient fluid in cup or pressure tank.
Too heavy fluid for suction feed.
Dry or worn packing or loose packing nut.
Plugged vent on suction feed cup.
Gun fluid inlet loose or not sealed/seated correctly.
Fluid hose or cup not fitted correctly to gun fluid inlet connector.
Gun is sucking air around the cup gasket. (siphon feed)
Fluid needle damaged or loose.
Fluid needle not seated or sealing correctly.
Air cap is damaged (off center) and lets air in around the thread.
There may not be enough material in the cup, and if you tip the gun excessively, the siphon tube sucks in air and causes sputtering. The material may be too heavy for the gun. Often there's an obstruction of dried finish somewhere — look for it. Check for a loose fluid nozzle, or a damaged or dirty fluid nozzle seat. A loose or cracked siphon tube may be the culprit. Look for a dried needle packing, or perhaps a loose or defective swivel hex-nut that connects the gun body to the cup.
Fill the cup 1/2 to 2/3 full, and don't over-tip the gun. Back out the needle packing hex-nut and put a drop or two of light oil on the packing (or replace the packing if it can no longer make a seal). Tighten the fluid nozzle, or remove it to clean the contact areas with thinner. Thin material that is too heavy to spray.
In regards to bubbles in the paint pot or cup -
Air leaks. Usually an air leak simply means the air nozzle is loose — tighten its retaining ring. Air leaks also occur if threaded connections are loose. Use Teflon tape on the gun's threaded air input before fastening your hose connector and on the threaded coupling between the gun body and the fluid nipple. Also, obstruction in the gun body fluid passageway causing the pot to become pressurized. A blocked vent hole in the pot lid. Finally, the needle-packing washer may be dry and worn, or the hex-nut cap that tightens the packing may be loose.
Try solving your problems with my tips. If that fails, your gun should be looked at by a professional gun repair company. Get a quote for repairs first, because you may well find your cost to repair and set right you gun problems, is close to or more than a purchase of a new gun. Older, well used and defective guns plus cheaper bought guns tend to be throw away items these days because of the cost of repairs and spare parts.
Hope you solve your problems,
Greg.