Have you tried the "goto cause" analysis option in SimVision?
You do need to have elaborated your design with "-access +rc" to enalbe the connectivity tracing (that's the +c bit).
Then in the waveform window you can put the cursor on a value you want to trace (doesn't have to be an X).
Next, right click on the signal's name and select "go to cause" from the pop-up menu.
This should then open the source browser, pointing a the line that caused the transistion to the value you're interested in.
You can repeat this to trace back as far as you wish; the time cursor automatically jumps forward in time to the transition, so all you have to do is keep "going to cause".
Hope this helps.