So I found out what the problem is. According to the guys at my LBS, there is a known problem with FSA cranksets that were made about 5-6 years ago (ironically about the same time mine was made!), where the single bolt that holds the non-drive-side crank arm onto the spindle is slightly too long and it bottoms out before it achieves proper pre-load. The bolt will feel tight, but will start to loosen ever so slightly, allowing the crank to slightly shift along the axis of the spindle. FSA actually sent out a bolt to whomever registered their cranks to fix this problem.
Additionally, when they took mine apart today, they commented that the bearings in the bottom bracket cup had forced themselves onto the taper of the spindle, and are basically press-fitted onto the crank itself, so they are removed from the bottom bracket cups whenever the crank is removed. I had noticed this the few times I took my crank apart, but thought that's how FSA's are designed, so didn't think anything of it.
Anyway, they had an Ultegra crank that didn't have any chainrings on it sitting around because a prior customer didn't want it, so they sold me that, new bottom bracket cups, and transferred my existing chainrings onto it, so I'm back on the road