Thanks to everyone for their help.  Please allow me to continue asking
questions toward my investigation.

Loren: Regarding clogged radiator, I can understand how that would
diminish cooling capacity, and perhaps only lots of wind at high speed
is enough to keep the engine cool with a clogged radiator.  But, how
does a clogged radiator cause bubbles in the expansion tank?

Alistair:  I have seen mention of testing for exhaust gas in the
coolant.  Can you describe the procedure?  Can I let the engine idle
with the expansion cap off (without making a mess)?  Or, can the
sniffer be placed in the overflow tank?

Bob: I have installed a new expansion cap.  I also had the cooling
system pressure tested a few months ago, and I replaced a hose that
was discovered to have a leak.

Thanks,
Travis