I only use software synths nowadays. Could this really be a CPU-issue? I mean upgrading to an Intel i7 processor was probably together with two SSD-disks the most effective change I made from the computer I had before with Sonar 8.5, with which I never had a jumping Now Time marker.

I'm running XP pro 32 on an i5 chip.... no jumpy issues here. Everything is in the box as well. 

Just tested and re-verified that the Pause/Break key will toggle Sonar from smooth cursor to jumping by measure. It is easy to hit this key by mistake.

Thank konradh! I have to check this too!

Pause/Break key! That's it. Thank you guys!

I find after a long session (1 or 2 hours) doing a lot of PRV MIDI editing thngs get a bit 'gluey', e.g. when I click on a particular note it sounds after a tiny delay instead of instantly, which is the norm.  I close everything down and restart and that cures it.  Bit of a pain though.