I doubt it is the pot life of the epoxy primer as 15 minutes should not be too much.
As for the gun, for the clear coat a high quality gun does make some difference, but not for most of us that only do a few paint projects...I have a kit of 3 guns I got off of TCP Global a few years ago, as long as you maintain them between use, they should be good for a long time.
http://www.tcpglobal.com/TCPG7000.ht...3#.XGsJ9I3ruUk
Do you have a good filter/dryer on your compressor and a moisture trap right at the gun? Moisture in the line can cause a ton of problems as will a loss of air pressure and volume.
Does your compressor have enough output to maintain 10-14 scfm at 40psi? If it can't keep up with that then your compressor could be an issue. That HF HVLP gun will likely be on the high end of that cfm range.