Hi Rebel-
Thanks again for your helpful post--let's see if it can get me my results. :]
Sorry if I sounded inconsistent in that last mail, but that's b/c the light guns *are* behaving inconsistently...

Exact sequence of operations (with *only* P1 gun plugged in):
1 - Boot Windows (800x600)
2 - Flip gun 1 switch to calibration. Screen goes into white cal screen w/out degaussing. Wave gun up & down, back & forth.
3 - Exit gun 1 calibration. Screen flips directly back to windows desktop w/out degaussing.
4 - Shoot around on windows desktop--gun behaves normally w/ decent accuracy
5 - Launch MaLa
6 - MaLa launches following command line: mame.exe area51a -offscreen_reload -autoframeskip -nomouse -lightgun , & mame.ini has the following input device options on: mouse, joystick, steadykey and the following off: dual_lightfun, offscreen_reload. No degaussing or forcing resolution to change from Win desktop.
7 - In Area 51, all shots fall on a diagonal line from top left to bottom right. Gun appears to be reading only "X" value--that is, the bullets move to follow my left-right position, but moving the gun up & down & firing results in bullets in the same place
8 - Pause Area 51 & do gun calibration again. White screen comes & goes normally w/ no degaussing.
9 - Unpause & play Area 51. Same diagonal shots.
10 - Go into Area 51 service mode (F2)
11 - Enter gun test.
12 - My MAME red & blue big crosshairs are *not* on-screen. Pressing F1 toggles them on & off. I have them off. Area 51 has a tiny gray crosshair in the center of the gun test screen & says "Aim each gun at crosshair below. Hold trigger until flashing stops." So, I point the gun @ the center crosshair, hold the trigger, screen flashes a few times, then stops & it says "DONE" The calibration routine that you describe doesn't seem to be applicable to Area 51--are you thinking of another game...?
13 - Exit gun test. Flip test switch (F2), play Area 51. Same diagonal shots...

To try to answer your other questions, the degaussing happens *almost* only in MAME. While running the ACT Labs test target program just now, out of 50-odd firings, 1 triggered a degauss. I wonder if my monitor sees the blank screen & sometimes thinks that it's lost its signal from the PC...?
I think I'll try to troubleshoot these one at a time. For now, I have just P1 gun plugged in & want to get it shooting properly in MAME. Then I'll add P2 gun into the mix.
I also tried Bang (run w/ same options as Area 51, except for no need for -autoframeskip, as it runs fast enough on my PC. Same results. Tried doing ACT Lab gun cal in game again, but no change.
I also wanted to try something "old", so I tried Cheyenne too. Ditto above.
Thanks for your help! I hope you've got an idea of what I should try! (where's the "praying" smiley anyway...

).
-Jason