Thanks JMG. It look like the new washer as a very slight raised rim along the edge on the side that faces the blade. On mine, that is worn out. I wonder if that ridge gives the washer a bit of "spring" to hold tight against the blade and arbor nut.
I read on some other forums that a thin card paper washer can also help. I am going to try that first, and if it doesn't do the job, I'll order a new washer.
The other irritant with this saw is that you need a very thin 1-1/4" wrench to hold the inner arbor nut. Very hard to find. There are guys selling these on eBay for $20-50! I might just get a regular wrench and grind it down.