I have the exact same problem. When the clamshell on my display is closed, the machine will display through the Dell monitor for an intermittent amt. of time before it goes in to powersave mode. some times it will stay connected for a long time, and some times it will "power save" after like 4 minutes. It's so frustrating. I have set it to never go to sleep. I know the mac knows what kind of monitor it is because I have a color profile for an E228WFP in my display preferences. I cannot find a driver for the monitor anywhere online. I have removed and reconnected. I have made energy saver adjustments in system preferences. I have changed the input on the monitor from DVI to AUTO. so far, the only thing that has fixed it (this time) is leaving the clamshell lid open.
Driving me nuts. Someone please help?