Okay, with the Bearhawk eGroup's help I figured out the problem. My big compressor is in my basement with a 50' hose connected to a pressure regulator/water separator attached to my work bench. I have one of those yellow coiled hoses running between the regulator and gun.
I tried a different setup and used a Porter Cable 6 gal compressor with a 20' hose connected directly to the gun with the pressure set at 100 psi. I almost over drove the first 470AD5-13 I set. It went as advertised with a 2-3 second burst. A few more practice tries and I was setting perfect rivets.
This has been the biggest hurtle I've had so far. My mistake was not getting this sorted out sooner. I've been dragging my feet finishing up the spars because of this issue.
I'm scratch building because I want to learn and so far I've learned a lot! Besides learning the correct set up to buck -5 rivets, I've also learned that if something doesn't seem right it probably isn't.
I can't express what a relieve it is to finally get over this hurtle. Now I'm looking forward to working on my spars again instead of dreading it.
NOTE: some good advice I received: "The hose size as well as the length can cause a problem. If you look at the couplers that go on the hose for your connection you will see that the passage for air to pass thru is about 1/4 inch on most couplers. I use Milton 1833 H style. They have a MUCH larger hole." and, "install a 6 gal or larger air tank to a tee between the regulator and the line to the rivet gun. The air pressure won't drop during the 3-4 sec burst. A portable air tank, re-plumbed, works great."
