Thanks everybody for the tips. I have tried EVERYTHING I can think of from all the posts and tips and my own knowledge, I have installed and re-installed SH3 and GWX 2, changed Nvidia drivers and tried most nvidia settings and NOTHING helps

. I would have blamed the card if I had problems with other sims (including SH4), but I dont...........so in a moment of desperation after spending the last day and many hours into the night I GAVE UP!!!!!!!!!....and this morning I went out and got a new card 8600 GT 512mb.............and you guessed it.........it all works fine now

.
I suppose I never will know what the problem was with SH3 and the 7300 GT card...............its a real shame I had to buy a new card, but pleased it all works well now.
Thanks once again everybody for your help
Cheers
S!