Refresh rate resets

[Niksko]

I have a Radeon HD 5870, and I'm using the restricted ATI Catalyst drivers. I can set my refresh rate to 85Hz, but then after a while (not simply after the screen saver comes on, but longer than that) the display resets to 60Hz (apparently the 'preferred' refresh rate), which looks terrible and tends to give me a headache. I then have to set it back to 85Hz.

I suppose this is a somewhat minor gripe, but if anyone could help me to fix it, I'd be very appreciative.

Thanks

-Nik