FS Time speeding up !!

Hi,I am having some strange problems recently whereby my FS time clock is running much faster than normal. When I check the Windows time clock, this too is running very fast but when I shut down Flight Sim all returns to normal!!I have now downloaded a time sync utility and have noticed that in just one minute, my time has increased by 18 seconds at least. I have checked my FS settings and the time is set to NORMAL.Has anyone any ideas as to what may be causing the problem.thanksKeith