France never breaks alliance [FIXED]

I'm playing VM as England on H/H and I started with an alliance with France. I have on marriage with them. I have left them alone so far this game, its now 1300. I've conquered HRE when the pope excomunnicated them, and I also whacked Milan.
They have been at war with Spain for most of the game and the cites near me are so ripe for the picking. But I have held back and conquered up North, just taken on Russia with their hard as nails armies.
Thankfully I have agood supply of swordsmen backed up by swiss pikeman and longbows, which you really need against their mobile armies.
I am so tempted to squash France but they have been so nice to me so far. Does anyone want to guess which turn they will stab me in the back.
I have kept to the popes wishes all game, always with a high rating by the pope, though diplomatically I am classed as dispicable becuase i jumped Poland for a few prize cites. I am just about to kick Venice out of italy and let them spat it out with the Byz while I keep France between myself and Spain and Portugal. They seem to be doing a good job of it.
France are excommunicated for picking on Spain.
Is this strange behaviour by France anything to do with VM game changes? I'm used to having to wipe out Scotland, then France, then the Danes before dealing with the rest of Europe.
I was finding thing a little too easy in Europe before I met Russia, perhaps Strengthening the HRE would be good as the Pope, Venice, Milan and Sicily all picked on them from the start before they had a chance to get some good units.

It depends whether you rush or take it moderately slow. Its not really possible to make the AI rush resistant without making them overpowering if they should go to war with you early on.

That said, alliances hold depending on whether you are a trusted ally and your relations with that faction. If both your reputations are quite high and you have good relations (which normally is the case with marriage alliances), the chances of backstab are very low, especially if they are engaged in another war. This is to prevent the AI from overstretching itself and spelling out its own doom.