I've had similar problems and it took a while before it dawned on me that the PC seemed to be receiving 'false' mouse clicks, effectively cancelling the menu call. Installing a new mouse fixed the problem.
I'm not guaranteeing that I've identified the fault with your system but changing the mouse is certainly worth a try. You can buy a perfectly acceptable conventional mouse for �2.47 from Tesco. Their optical mouse (which is the best mouse I've ever used) costs just �5.47. (Another couple of pounds will buy you the wireless version).
Everyone should keep a spare mouse handy because they don't last forever and they always go wrong just when you've got something really urgent to do on the PC. So, even if it doesn't fix your problem, the small expenditure required to try my suggestion won't be wasted.
Chris