PROBLEM SOLVED!!
I replaced the 1/4" quick-disconnect couplers with 3/8" couplers and now the impact gun spins the lug nuts off without even wheezing! No coughing, no chugging, no chattering, just BRRRRRRR, and the nut is off and spinning around on the garage floor like a toy top! SWEET!
A little more detail......
I replaced the quick-disconnect couplers at the compressor, both ends of the air hose, and the inlet of the impact gun.
I did not replace the crimped-on, male-threaded ends of the 3/8" hose. They are still 1/4 in-18 NPT.
The 1/4" quick-disconnect couplers have a ID of .200. The 3/8" couplers have an ID of .275 (see picture). If I did the math right, that is a 77% increase in CFM air flow!! All of you that said "get more air to the gun", were exactly right!
Testing both the 1/4" and 3/8" couplers, the PSI at the gun remained about the same - 110 psi with trigger NOT pulled. Approx 60 psi with trigger pulled. Even though the PSI stayed about the same, the increased air flow (CFM) did the trick.
Thanks to all for helping me out.
I'm gonna go play "NASCAR pit crew" just because I can! hahaha