Close Mach then look in the Mach3 folder on your computer and you will see DriverTest.exe, double click it and let it run, after a few seconds it will start running and you will see a line, if it is spiky then thats not so good unless its only small spikes. Also make sure you try it at the same kernel you are using for Mach by clicking on the frequency in the driver test and then clicking start timer, it will then start checking at that frequency.
Also worth saying, f you have the kernel set higher than you need in Mach lower it as that may help.
Hood