Anybody else have the problem in X3d with the position marker randomly jumping back to the beginning of the song when you are working elsewhere in your song?

Yes it does happen to me too, but it's always during a key press, and I've never been certain my fat fingers weren't also hitting something else too...

It's very annoying and yes, that's always your first though, was it my fingers, but when I know I was nowhere near the W key?

This is an annoying issue. It happens when I use Ctrl+Shift+PageUp/Down (all on the right side of the keyboard) to move between markers (and I have no marker at the beginning of the song). I haven't found a way to reproduce it successfully using a default project template so I can't report it to CW, though. 

It's done it a few times now whilst working in melodyne, when I press the spacebar.

Jlien X
This is an annoying issue. It happens when I use Ctrl+Shift+PageUp/Down (all on the right side of the keyboard) to move between markers (and I have no marker at the beginning of the song).

Possibly you're inadvertently hitting Ctrl+Home on occasion?.
 
This won't help avoid that, but just incidentally, I found that I never used Page Up/Down to page tracks, so I re-mapped Page Up/Down to previous/next measure, and Ctrl+Page Up/Down to previous/next marker. Saves some keystrokes. I try to avoid combos where I have to hit more than one modifier for the most common tasks.

+1 spacebar. sometimes sonar stops responding momentarily...

Ok it just happened to me now, I was doing and edit in take lanes midway through the project, had split a clip and nudged the timing. When I hit the space bar for a listen it went to the begining of the project.  No fat fingers involved.

Houston, we have a glitch!

Sixfinger
Ok it just happened to me now, I was doing and edit in take lanes midway through the project, had split a clip and nudged the timing. When I hit the space bar for a listen it went to the begining of the project.  No fat fingers involved.

Where was the Now time while you were editing? It's quite possible to scroll the track view to the middle of a project and do edits like that while the Now time remains at time zero or somewhere else.
 
I've never had an unexpected/unexplained relocation of the Now time that I can recall.