Windows 7: Sleep function

Ever sunce installing Windows 7 last October I have been unable to get the Sleep function to work as it should, that is ti say that on seleecting Sleep in the Start Menu the display goes blank momentarily but re-appears after some 10 seconds or so.
Since installing I have tried unsuccessfully on many occasions to correct this and understandable]ty have been reluctant to resort to re-istalling Windows to resolve this matter. However when,, for anorther reason this, over a period of time, became necessary, it still did not resolve this problem..
On a trial and error basis I have discovered a workaround whereby if I select a screensaver setting with a minimum period of one minute before displaying following this then the Sleep function does operate a it should. The only drawback with this however I have found is that periodically Windows 7 decides of it's own accord to change the
screensaver setting selected to None and then once again the Sleep function faills tW operate.
Any suggestions for resolving this matter would be much appreciated.