Don't blame you mate, back when I was racing nitro 1/10th on road, I had a couple of runaways on the spektrum, to this day I have no idea why, or why the failsafe, er, failed. I don't trust it either. I use a cheapy ansmann one as a backup, but I've no idea how good it is, having never had a signal loss since I installed it. I'd say probably any would do, go for a reputable brand for peace of mind...
Something like this maybe?
http://www.amainhobbies.com/product_...icro-Fail-Safe