Doom II crashes and screen rotation changes (Win 7)

[volvotgb13]

So i've finally got my old friend Doom II up and running through Steam and I find it regularly crashes especially (but not always) when I'm in full kill mode (ie being attacked by mucho baddies) and hitting CTRL and ALT at the same time as moving - aka shooting and sliding sideways.

The screnn goes blank and I have to ALT/TAB to switch into windows , fix the screen rotation and usually have to close dosbox and restart the game. This is pissing me off so any ideas on how to sort this would be appreciated. I've sent an email to Steam support but 5 days later and no reply so I think I'm almost on my own here.

[volvotgb13]

forgot to add, I'm using classic controls