With RE050As on the front still, I wonder why...
I've only had Zed's with matching brands of tyres front and rear, so excuse me if this sounds stupid but what bearing would the front tyres have on the rear? It's an honest question as I'm curious.
Stones will no doubt have better grip than the Falkens, hence it seems tail happy. If you had the Falkens all round, the car would lose grip at overall less speed than having all Stones, but all tyres would have the same grip, so rather than being tail happy, it would drift more, but would be more balanced over all. So while you have less total grip with Falkens, it would be balanced.
Also made worse by the fact that you probably havent worn off the tyre mould release agent yet. That takes a good few hundred miles depending on how you drive.