Until now the only softsynth that is crashing X1 consequently (and taking X1 down with it) is Yellow Tools Independence Free (I guess this is true for the paid version too). The circumstances to make it crash is not fully sought out by me, but until now it seems to crash only if an instance of one of the Studio Instruments is present. When I start a "clean" project Independence has no problems.

I will experiment a little bit with it to make sure what combination of circumstances makes this synth crash.

Anyway - apart from Independence I do not have any problem with any other synth. I also do not have any other crashes with X1. The "Independence problem" is the only thing that made X1 crash until now.

To add to the list of people having crash problems with X1, put my name on the list. So I have gone back to sticking with Producer 8.5 in the mean time until I read enough about X1 having more stability. Working on the same project in 8.5.3 has had no crashes. (just some popping one time that I wrote about in the Producer forum - and thats ok now).
 
I thought the problem was over when I downloaded the X1a fix....it happens rather randomly so I have not been able to clearly say its caused by one thing or another. I think one time I mistakenly tried to drag a clip from one track layer to another, so this was apparently such a no no that it crashed....I think another time was right in the middle of my trying to work with Session Drummer 3 and the step sequencer view......
 
Also - is it common after a crash, for it to take several minutes (without rebooting) for the program to re-load no matter how many times you click on its icon to open it? I am not techy enough to know...its as if the program has to re-group itself on the drive before it will reload??!