Warren:
I fought the same problem with exactly the same symptoms. Only during light braking. In my case the cure was to change and go to a Hawk pad.
You can find the thread on the ERA site here on Club Cobra, just go back through the threads. Lots of information on what people tried to cure the problem. Some worked some didn't.
Changing to the Hawk pads cured my problem after trying compounds, different Willwood pads, shims, lubes and several other fixes either stand alone or in combination.
Check with
http://www.hawkperformance.com/performance/hps.php for their pads and you will probably find one to match whichever caliper you're using.
DonC