Well, no one wants to factory reset as their first option. Haha.
A buddy of mine has an X and has not had the issue you mentioned. I mean, I'm sure it's not likely a coincidence that it started happening after your install, but, it's a possibility.
Do you have another battery? I'd try swapping it out, probably wont help, but, worth a shot.
This is kinda a weird idea, but, just a thought. When I turn off my X2 and plug it in it goes to a charging screen where it displays the a battery charging indicator with percentage. When you turn your phone off, is there enough time to plug it in before it turns itself back on? Maybe try plugging it in and see if you can get to this screen instead of having it turn itself on. I'm not sure if the original X does this, but, if it does, might be worth a shot.
If you can get to this screen I mentioned, maybe try a couple of things. Turning it on while plugged in, then off, then unplug it, see if it stays off. If that doesn't work, maybe try unplugging it at this screen, then when it stops displaying this screen, see if it stays off.
These aren't really solid ideas, just things I would do to mess around with the phone while hoping someone else has the same issue and/or a fix.
But, srkmagnus might be right, you might need to do a reset. :'( I'd still hold off on one though since it's not really the hugest issue. Worst case you really need it to be off for some period of time, you can pull your battery until you have a fix or factory reset.