This mostly happens because OpenMPT does not double-buffer the pattern display, which does not play well with the compositing window manager used by default in Windows 7.
There are several factors that may come into play why it just started happening for you, e.g. because the compositing window manager was not used (on Windows 7, it can be disabled if you are not using Aero, but Aero designs require it) or because you used a different audio configuration with a different period setting, which directly influences how often the display is updated.
Ultimately we need to start double-buffering the pattern editor (just like the instrument and sample editor), not doing it is a relict of old times, but until then, I can only recommend trying to experiment with the period settings in the sound card settings or disable the compositing window manager if you use the "classic" Windows look anyway.