I have experienced that as well and also tried to identify some pattern but could not. For me it happens mostly when I work really zoomed in on some part, then scroll wheel stops responding, pan (right click) is super slow, etc… It might have something to do with where the camera is looking, maybe some action shifts it far far away. John_TYler said this in a previous post:
" 1. The camera scheme is that it points at a particular location, and rotation and zoom is done based on that location. The camera’s fixate point can be moved perpendicular to the direction it’s viewing with panning (right click and drag). If you rotate to a top-down view, pan the camera over the area you’re interested in, then rotate to a side view and do the same, it will put that fixate point at the heart of the area you’re interested in. That will make the rotate and zoom center on the bit you want."
Sometimes it helps, but on times where the zoom is super slow and the zoom wheel seems to do nothing, I just exit and reenter the project, auto-save did not fail me so far.