I had similar problems as yourself . What I had to do was sacrifice the better detailed graphics and set the game to these settings . Some courses are still hit and miss but the game is at least better to play .
Here is a screenshot of my settings .
Just to give you an update Mark . I did as suggested and reinstalled Steam . Saved my courses . Must have umpteen copies now . Everything ok again . Not sure what happened but heyho joys of the pc world .
Thanks again