I have had my 4890 overclocked running Milkyway@Home every day as well as using it for gaming. I have not experienced none of the issues you have described. However, when I pushed it too far, it would BSOD all day. The issue is either a faulty card, some issue with driver installation (i.e., didn't clean your NV drivers before you installed the card), or insufficient voltage at 2D settings, perhaps a poor PSU/connection.
As far as monitor power saving mode, I can tell you that it doesn't work for either 8800GT or 4890 I have on my Westy 37 inch or my friend's Sharp 32 inch using a DVI connection. This feature only worked for me on those cards using an VGA connection. So it's a common issue with NV/ATI.