Just wanted to add - this is the kind of mouse that doesn't need cleaning, so it's not that. Plus, I checked the plugs and they seem secure.

well if it were a battery operated one I would suspect the need of new batteries. Or that it's line of site to it's transmitter receiver was blocked.
Have you checked your mouse settings in control panel to see if possibly some settings went awry?

Check the surface you have it on, sometimes they do not work well on very shiny or glass like surfaces.

What kind is it exactly? brand and type.

I have seen this issue before and if it is not a mouse issue I would consider the following:

If it is an older system which has its resources pushed to the limit it could be one too many operations in the background.

Even on a new system I would consider checking the Startup and a spyware check or two wouldn't be a bad idea either as something may be running.

DA

Thanks for your help. I turned off the computer and turned it on again and it seems to be ok now. Go figure!

Well by doing that you cleared the ram and reset all the seetings many problems can be solved by a simple turning the computer off & on (Rebooting)..