Frequent crashes and lockups with new X3 Studio installation

Hi,
I ran X3 Studio happily for a year or so, then I had to reinstall it after a hard disk failure. Now it's crashing or freezing very frequently - about every two minutes on average.
I installed it as 64-bit (PC is Win 7-64 with i7 and 8 GB RAM). I've put it on the D drive, as my C drive is an SSD and getting a bit full.
I'm using it with a Focusrite 18i 20, with the ASIO interface.
Can anyone make a suggestion?
Thanks,
Andrew