It turns out - and hence my OP question - that one of my Titan extruders was in fact skipping. I thought at first the motor wasn't strong enough .. or that there was a clog in the nozzle - or that .. well.. 1/2 a dozen other reasons I was checking.
As it was .. I just didn't have the current set correctly. It was set at 400mA (it probably came from a copy/paste doing the config) .. and it really should have been 850mA
Once I made that adjustment - so far, it's been working great.