Go into your system files/device manager and see if you have more than one mouse listed.......if so delete all others except the one you use regularly. I had that issue when I changed mice about 6 mos. ago.
Then make sure all of your drivers are up to date for mouse, flash, etc. and also make sure your page file memory is set to 2500 mb or higher. This insures your computer won't bog down while playing.
If your internet connection is slower than usual.....this will also effect your game when competing, but you won't notice it much when playing alone.
I guess i should ask if it other applications or programs are not working as they should be. Or if it is only the dart playing that is affected.