Well I found things improved after I had installed RS Patch 2.1a. See
http://www.twcenter.net/forums/showthread.php?t=405358
I still get an occassional CTD during a battle. Usually when things get really hectic and I'm clicking and scrolling around the battle map a lot. But its rare enough now to be just a

moment rather than a game killer.
I've also noticed that sometimes during a battle I get the default 'windows fatal error' sound go off in the background, but usually nothing actually happens although I instantly freeze what I'm doing whenever I hear it just in case and hold my breath until the mouse starts moving again.