[j-nsp] M20 stops processing icmp/snmp/local traffic

[billp]

M20, running 5.6R2.4.  Has been up for 143 days, no issues.

Suddenly, it stops processing some traffic:
	- snmp queries to the router time out
	- icmp to the router times out
	- icmp through the router times out sometimes
	- ssh to router times out
	- router cannot ping some local devices connected to it
	- 4 of 25 BGP sessions dropped and will not re-establish

Strange thing is, other traffic seems to be passing through
it fine.  We can reach devices through the router, and we
are passing 700+ Mb/s traffic through the various interfaces.

show pfe * all looks good, no obvious problems

sho chassis ssb shows the ssb processor at 70% (about 20% higher
than normal.)  Our 40-50% CPU is usually caused by sampling, so
I disabled all sampling.  No change.

top doesn't show any processes running crazy or hogging cpu,
load average on RE is 0.20ish.

Any other thoughts on what to look for or try?  

thanks
bill