We do not have a very good computer, so sometimes if too many ZynAddSubFX instruments are being used, or especially if some of the more demanding VSTi plugins that are out there are are being used in a project, it is too much for this computer to handle.
In the previous version of LMMS, there was also a bug that sometimes caused the processor usage to go up and down, even when LMMS was not playing anything. I do not know if that bug was fixed or not, but haven't seen it since updating to the latest version.
^There are other threads about this kind of problem too. Look through them and see what people who are way smarter and more experienced than me are saying in them. It might help.
