It was not a PS but a Microsoft problem.
There is a wmpnetwk.exe process which runs in the background; a windows media player network access process. Every time I opened or saved or did something RAM-intensive in PS, this tiny RAM-sucker blew up from ~ 9 MB RAM use to over 2 GB! without doing something useful. What was strange is, that it wasn't displayed in the normal 'process' tab in the task manager (then I would have seen the problem earlier). Only when I opened the detail views in the resources tab in the task manager I found it.
I disabled the wmpnetwk.exe permanently - only shutting down is no help, it is restarted automatically within a few seconds.

Together with PS this process used nearly my entire RAM capacity of 4 GB; I'm not sure, but I think that wmpnetwk.exe is granted priority by Windows 7, because PS went down to a few hundred MB RAM use, while wmpnetwk.exe became bigger and bigger.

I hope this information can help others in the future.