A few folks suggested trying a new reservior cap.  So I swapped
with one from another vehicle - that cap is only a year old - but it's
still doing the same thing.  So I am convinced that it is combustion
gases in the coolant and need no sniff test.  I also got a tip that if
it's the big O-ring, that sometimes retorquing the head bolts fixes
it.  So I'll try that next.  If that doesn't help, then it's time to pull the
heads for new gaskets and maybe new heads if they fail the
pressure test.

Thanks everyone!