Thank you for the help!
I reinstalled first, but then decided to try it with the tracing build you supplied. So far, though my use has been somewhat limited the last days, UR has run surprisingly stable. Will report back when I can make a more definite statement.
As for what has changed: I installed Vista
Other than that, it's hard to come up with anything specific. Since it's a new setup, things have changed pretty rapidly and lots of new software has been installed at various points in time. I can't even tell for sure whether problems started immediately with the OS change or rather appeared later on.
No other computer to try, unfortunately.
BTW: A minor, easily reproducable bug I found in UR 3.1.1 is when you have multiple tabs open and close the second-to-last tab using the 'x' at the right (not the "Close Tab" entry from the popup menu), an empty tab with the original tab's name will appear to the right. Now clicking on the initially last tab (now to the left) will restore the initial tab setup but with the tab names exchanged. This can be repeated indefinitely.