...Tackling the problem from the other end, I once sawed/filed a groove in a steerer thread without one, to be able to use a tabbed washer, in an attempt to correct just such a problem as OPs- It didn't work- the headset still worked loose... although admittedly the 'tab' on the washer was not very good. That was an alloy headset- what was left of the threads, on the locknut especially, wasn't exactly communicating the message; "tighten me to f-ck!" The bottom cup was a bit looser fit in the headtube than what I was familiar with, too. In the end I replaced the headset with a cheap steel one and it's been trouble-free (so far) ever since.
mattr- my avatar may jog your memory! Headset is 32mm, fixed cup is 36mm. You must have had three spanners?