Hi dave4sal, so does this always happen, have you installed some other software or hardware before this started happening, does it happen with any other internet use?
You see dave4sal, their could be lots of potential problems and solutions. It would help if you could post your PC specs ( Motherboard, Ram, CPU, Gpu, OS-32bit/64bit etc ) as all this information as well as a better explanation of problem eg. does it affect only when typing email in yahoo? have you another email client to test with ( like Google gmail? )!
Update thread and will try to help solve problem, NEED more information though dave4sal.