cant say for certain but it could be your monitor on its way out, check the tightness of the connection between monitor and computer, also might be your graphic card needs and re-seating, just to get a good connection. if possible try a different PCI slot.
get someone to help if it scares you, or take it back to where you had the upgrade fitted and ask them to check it out
doubt if it will be the mouse but it could be possible. check all wires to the back of your pc, its easy for one of them to work loose.