Bill is probably right with the seals being worn. If that was the case 15 years ago they have only become worse with age. If it is a siphon feed make sure the vent is clear in the top of the cup. If it becomes clogged air can't get into the cup and the gun won't be able to siphon the paint out of it. If it is clear and the gun still sputters, you have an air leak somewhere that's interupting the siphon and making it sputter. Either way a good cleaning and re-packing should bring it back up to snuff.

Good luck,
Dan
www.gocolours.com