Try removing all plasmoids you added, just use a basic desktop.
or
Try a new .kde4 folder (keep the old one, just rename it .kde4_old) see if you still have the problem. If not then the problem is in your settings and not necessarily a fault in plasma.
For me the process that is constantly at around 50% is Plasma. That’s it just plasma, so I don’t know if the widgets are encompassed in that particular process.
The calendar did it for me. I removed it and the cpu immediately went down to its normal mode.
The question now would be why the calendar? Before upgrading to 4.2.2 everything was fine. Oh well, I can live without the calendar for now. Hopefully it would get fixed.
I’m repeating a reply from another thread. I had this problem and removing the calendar widget solved it. But, I have a low spec machine running 11.2 and kde 4.2. That machine has no problem with plasma and the calendar widget.
I started having the problem about the time I noticed that the nvidia splash screen on this pc no longer said “beta.” It must have updated my legacy driver. It could be that, since the other pc has on-board intel graphics.