I've always had occasional playback issues on this system, which though relatively high-spec is typically running multiple desktop apps plus several resource-hungry background processes. All I can say so far is that the jump/stutter behavior seems a little worse with 12.1 than prior releases. Killing some background processes did seem to alleviate the issues but not solve them completely. Looking at task manager there's no evidence of conflict for processor time (iTunes is consistently using around only 2%) or memory - which reinforces the thought that the issue is related to data access speeds and demands.
I didn't notice any issues on the 1st system I upgraded, though it is 32-bit Win 7 and has a USB3.0 external drive. The interesting test will be on the last one I will be installing 12.1 since it is 64 bit but uses USB 3.0 for connection to its library and media data storage.
It would be interesting to know whether anyone is noting this degradation in playback having upgraded to 12.1 32-bit, and the degree to which having library/media files on faster or slower storage makes a difference.