Since your system is a lay system TO, you'd have to do the reverse of what aussie has said. Previously have your lays been coming 2nd and 3rd and have now just taken their turn at winning or were they previously unplaced and now started winning?

Also are there any logical extensions to your system that would rule out a portion of your losers without taking away any (or none) of your winners? Obviously adding a backfitted rule like must not have been within 2.3 to 3.7 lengths of the winner last start is not appropriate, but you may find something logical that fits with your system which rules out a number of the losses.

In saying that, i'm obviously not on the same level as some of these others guys but I think if you took a combination of UB's, CP's and Aussies idea's you'd find out if you need to keep it or can it.