I'm guessing that your external display is a flat screen TV and may not be larger than about 36". When connecting PCs to a TV like this, the PC's video system and/or TV prefer to run in 1366x768, which is the TV's "native resolution". This happens with my Sony 32" HDTV using the HDMI connection, and a discrete video card in a desktop PC. I can set it to 1920x1080, but that setting does not fit the screen correctly, it extends beyond the physical size of the screen. Also at 1920x1080 on the TV, the refresh rate is forced to 30Hz, rather than 60Hz.
According to my TVs manual and a warning from my video cards software, running at resolutions other than the recommended or native settings when used with a PC can actually damage the TV. There is apparently some difference in the PCs signal and the signal from standard video devices, I don't understand the details regarding what that is.
It would not surprise me if the control panel software is defaulting to the native resolution due to the potential damage problem, or simply because that is the TV's native resolution. I don't own a laptop like yours, so I am unfamiliar with it's video control panel, but when you choose 1920x1080, is there an Apply or Save Changes button in the window? That might save it for you, or do you do that now and it keeps getting reset?
You can also try setting this in Windows, I know you can do this in Windows 7 but I forget if you can in Vista. From the Control Panel, choose Display and then Adjust Resolution or Change Display Settings. I don't know if this will lock your resolution in with your PC, but it's worth a try.
Sorry for the delay in responding. My display is a 37" LCD and it supports 1920x1080. Before the update to my Intel Control Panel, everything worked fine- I had 1920x1080 resolution when using my external display as part of an extended desktop. Now I can get the display to 1920x1080 only my selecting this resolution manually everytime I try to use the display. So its the new intel control panel that won't 'remember' the 1920x1080 setting. The notebook is attached to the display via an HDMI cable.
I have the display attached to a second PC and it works perfectly in 1920x1080 mode- the display is the only screen for the PC as is attached via a VGA cable.